[RFC PATCH] mm: bypass swap readahead for zswap

[RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.

zswap is the same kind of in-memory, synchronous backend as zram, not a
swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
swapin_readahead().

Here are the results from bypassing readahead for zswap too: it was
measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
off, on Sapphire Rapids and 3 iterations.

  768M memcg (sustained swap thrash):
    metric                 mm-new    + bypass    delta
    build time (s)          405.0       341.7    -15.6%
    zswap-in (GB)            79.5        53.0     -33%
    zswap-out (GB)          144.8       115.6     -20%
    swap readahead (pages)  6.79M       0.45M     -93%
    swap_ra hit (%)          72.1        89.9     +18pp

  1G memcg (light pressure, build not memory-bound):
    metric                 mm-new    + bypass    delta
    build time (s)          177.7       176.0    ~same (no regression)
    zswap-in (GB)            10.2         7.5     -26%
    zswap-out (GB)           27.7        25.1      -9%
    swap readahead (pages)  1.07M       0.08M     -93%
    swap_ra hit (%)          68.6        87.2     +19pp

The gain is from no longer prefetching pages that are pointless for an
in-memory backend: readahead inflates anon residency and thrashes the
page cache (file pages get evicted and re-read), lengthens each fault by
synchronously (de)compressing a cluster of neighbours, and adds
compression traffic when those extra pages are reclaimed.

Bypassing swap readahead for zswap therefore makes sense.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
---

- This bypass originally comes from Usama's series that implements
  large folio zswapin: while working on improving this series, I noticed
  the gains I got only came from the bypass of readahead.

 include/linux/zswap.h |  6 ++++++
 mm/memory.c           |  5 +++--
 mm/zswap.c            | 11 +++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index 30c193a1207e..b6f0e6198b6f 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
 void zswap_folio_swapin(struct folio *folio);
 bool zswap_is_enabled(void);
 bool zswap_never_enabled(void);
+bool zswap_present_test(swp_entry_t swp);
 #else
 
 struct zswap_lruvec_state {};
@@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
 	return true;
 }
 
+static inline bool zswap_present_test(swp_entry_t swp)
+{
+	return false;
+}
+
 #endif
 
 #endif /* _LINUX_ZSWAP_H */
diff --git a/mm/memory.c b/mm/memory.c
index ff338c2abe92..5aa1ea9eb48a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	if (folio)
 		swap_update_readahead(folio, vma, vmf->address);
 	if (!folio) {
-		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
-		if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
+		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
+		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
+		    zswap_present_test(entry))
 			folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
 					    thp_swapin_suitable_orders(vmf) | BIT(0),
 					    vmf, NULL, 0);
diff --git a/mm/zswap.c b/mm/zswap.c
index 761cd699e0a3..5b85b4d17647 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
 		>> ZSWAP_ADDRESS_SPACE_SHIFT];
 }
 
+/**
+ * zswap_present_test - check if a swap entry is currently backed by zswap
+ * @swp: the swap entry to test
+ *
+ * Return: true if @swp has a zswap entry, false otherwise.
+ */
+bool zswap_present_test(swp_entry_t swp)
+{
+	return xa_load(swap_zswap_tree(swp), swp_offset(swp));
+}
+
 #define zswap_pool_debug(msg, p)			\
 	pr_debug("%s pool %s\n", msg, (p)->tfm_name)
 
-- 
2.54.0

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Kairui Song]

On Wed, Jun 24, 2026 at 3:59 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>
> zswap is the same kind of in-memory, synchronous backend as zram, not a
> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
> swapin_readahead().
>
> Here are the results from bypassing readahead for zswap too: it was
> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
> off, on Sapphire Rapids and 3 iterations.
>
>   768M memcg (sustained swap thrash):
>     metric                 mm-new    + bypass    delta
>     build time (s)          405.0       341.7    -15.6%
>     zswap-in (GB)            79.5        53.0     -33%
>     zswap-out (GB)          144.8       115.6     -20%
>     swap readahead (pages)  6.79M       0.45M     -93%
>     swap_ra hit (%)          72.1        89.9     +18pp
>
>   1G memcg (light pressure, build not memory-bound):
>     metric                 mm-new    + bypass    delta
>     build time (s)          177.7       176.0    ~same (no regression)
>     zswap-in (GB)            10.2         7.5     -26%
>     zswap-out (GB)           27.7        25.1      -9%
>     swap readahead (pages)  1.07M       0.08M     -93%
>     swap_ra hit (%)          68.6        87.2     +19pp
>
> The gain is from no longer prefetching pages that are pointless for an
> in-memory backend: readahead inflates anon residency and thrashes the
> page cache (file pages get evicted and re-read), lengthens each fault by
> synchronously (de)compressing a cluster of neighbours, and adds
> compression traffic when those extra pages are reclaimed.
>
> Bypassing swap readahead for zswap therefore makes sense.
>
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
>
> - This bypass originally comes from Usama's series that implements
>   large folio zswapin: while working on improving this series, I noticed
>   the gains I got only came from the bypass of readahead.
>
>  include/linux/zswap.h |  6 ++++++
>  mm/memory.c           |  5 +++--
>  mm/zswap.c            | 11 +++++++++++
>  3 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 30c193a1207e..b6f0e6198b6f 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>  void zswap_folio_swapin(struct folio *folio);
>  bool zswap_is_enabled(void);
>  bool zswap_never_enabled(void);
> +bool zswap_present_test(swp_entry_t swp);
>  #else
>
>  struct zswap_lruvec_state {};
> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>         return true;
>  }
>
> +static inline bool zswap_present_test(swp_entry_t swp)
> +{
> +       return false;
> +}
> +
>  #endif
>
>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index ff338c2abe92..5aa1ea9eb48a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         if (folio)
>                 swap_update_readahead(folio, vma, vmf->address);
>         if (!folio) {
> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> +                   zswap_present_test(entry))

Hi Alexandre

Thanks for the test and patch, very interesting idea.

> diff --git a/mm/zswap.c b/mm/zswap.c
> index 761cd699e0a3..5b85b4d17647 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>                 >> ZSWAP_ADDRESS_SPACE_SHIFT];
>  }
>
> +/**
> + * zswap_present_test - check if a swap entry is currently backed by zswap
> + * @swp: the swap entry to test
> + *
> + * Return: true if @swp has a zswap entry, false otherwise.
> + */
> +bool zswap_present_test(swp_entry_t swp)
> +{
> +       return xa_load(swap_zswap_tree(swp), swp_offset(swp));

Better check zswap_never_enabled first to avoid a xa_load if not needed.

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Nhat Pham]

On Wed, Jun 24, 2026 at 3:31 AM Kairui Song <ryncsn@gmail.com> wrote:
>
>
> Better check zswap_never_enabled first to avoid a xa_load if not needed.

+1.

Maybe also xa_empty() when we're at it? :)

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Hi Nhat,

On 6/24/26 19:43, Nhat Pham wrote:
> On Wed, Jun 24, 2026 at 3:31 AM Kairui Song <ryncsn@gmail.com> wrote:
>>
>> Better check zswap_never_enabled first to avoid a xa_load if not needed.
> +1.
>
> Maybe also xa_empty() when we're at it? :)


Yes I'll add that too.

Thanks,

Alex

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Hi Kairui,

On 6/24/26 12:30, Kairui Song wrote:
> On Wed, Jun 24, 2026 at 3:59 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
>> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
>> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>>
>> zswap is the same kind of in-memory, synchronous backend as zram, not a
>> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
>> swapin_readahead().
>>
>> Here are the results from bypassing readahead for zswap too: it was
>> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
>> off, on Sapphire Rapids and 3 iterations.
>>
>>    768M memcg (sustained swap thrash):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          405.0       341.7    -15.6%
>>      zswap-in (GB)            79.5        53.0     -33%
>>      zswap-out (GB)          144.8       115.6     -20%
>>      swap readahead (pages)  6.79M       0.45M     -93%
>>      swap_ra hit (%)          72.1        89.9     +18pp
>>
>>    1G memcg (light pressure, build not memory-bound):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          177.7       176.0    ~same (no regression)
>>      zswap-in (GB)            10.2         7.5     -26%
>>      zswap-out (GB)           27.7        25.1      -9%
>>      swap readahead (pages)  1.07M       0.08M     -93%
>>      swap_ra hit (%)          68.6        87.2     +19pp
>>
>> The gain is from no longer prefetching pages that are pointless for an
>> in-memory backend: readahead inflates anon residency and thrashes the
>> page cache (file pages get evicted and re-read), lengthens each fault by
>> synchronously (de)compressing a cluster of neighbours, and adds
>> compression traffic when those extra pages are reclaimed.
>>
>> Bypassing swap readahead for zswap therefore makes sense.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>
>> - This bypass originally comes from Usama's series that implements
>>    large folio zswapin: while working on improving this series, I noticed
>>    the gains I got only came from the bypass of readahead.
>>
>>   include/linux/zswap.h |  6 ++++++
>>   mm/memory.c           |  5 +++--
>>   mm/zswap.c            | 11 +++++++++++
>>   3 files changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
>> index 30c193a1207e..b6f0e6198b6f 100644
>> --- a/include/linux/zswap.h
>> +++ b/include/linux/zswap.h
>> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>>   void zswap_folio_swapin(struct folio *folio);
>>   bool zswap_is_enabled(void);
>>   bool zswap_never_enabled(void);
>> +bool zswap_present_test(swp_entry_t swp);
>>   #else
>>
>>   struct zswap_lruvec_state {};
>> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>>          return true;
>>   }
>>
>> +static inline bool zswap_present_test(swp_entry_t swp)
>> +{
>> +       return false;
>> +}
>> +
>>   #endif
>>
>>   #endif /* _LINUX_ZSWAP_H */
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ff338c2abe92..5aa1ea9eb48a 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>          if (folio)
>>                  swap_update_readahead(folio, vma, vmf->address);
>>          if (!folio) {
>> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
>> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
>> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
>> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
>> +                   zswap_present_test(entry))
> Hi Alexandre
>
> Thanks for the test and patch, very interesting idea.
>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 761cd699e0a3..5b85b4d17647 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>>                  >> ZSWAP_ADDRESS_SPACE_SHIFT];
>>   }
>>
>> +/**
>> + * zswap_present_test - check if a swap entry is currently backed by zswap
>> + * @swp: the swap entry to test
>> + *
>> + * Return: true if @swp has a zswap entry, false otherwise.
>> + */
>> +bool zswap_present_test(swp_entry_t swp)
>> +{
>> +       return xa_load(swap_zswap_tree(swp), swp_offset(swp));
> Better check zswap_never_enabled first to avoid a xa_load if not needed.


You're right, I'll add that in the v2.

Thanks,

Alex

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[David Hildenbrand (Arm)]

On 6/24/26 09:55, Alexandre Ghiti wrote:
> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
> 
> zswap is the same kind of in-memory, synchronous backend as zram, not a
> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
> swapin_readahead().
> 
> Here are the results from bypassing readahead for zswap too: it was
> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
> off, on Sapphire Rapids and 3 iterations.
> 
>   768M memcg (sustained swap thrash):
>     metric                 mm-new    + bypass    delta
>     build time (s)          405.0       341.7    -15.6%
>     zswap-in (GB)            79.5        53.0     -33%
>     zswap-out (GB)          144.8       115.6     -20%
>     swap readahead (pages)  6.79M       0.45M     -93%
>     swap_ra hit (%)          72.1        89.9     +18pp
> 
>   1G memcg (light pressure, build not memory-bound):
>     metric                 mm-new    + bypass    delta
>     build time (s)          177.7       176.0    ~same (no regression)
>     zswap-in (GB)            10.2         7.5     -26%
>     zswap-out (GB)           27.7        25.1      -9%
>     swap readahead (pages)  1.07M       0.08M     -93%
>     swap_ra hit (%)          68.6        87.2     +19pp
> 
> The gain is from no longer prefetching pages that are pointless for an
> in-memory backend: readahead inflates anon residency and thrashes the
> page cache (file pages get evicted and re-read), lengthens each fault by
> synchronously (de)compressing a cluster of neighbours, and adds
> compression traffic when those extra pages are reclaimed.
> 
> Bypassing swap readahead for zswap therefore makes sense.
> 
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---

[...]

>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index ff338c2abe92..5aa1ea9eb48a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	if (folio)
>  		swap_update_readahead(folio, vma, vmf->address);
>  	if (!folio) {
> -		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
> -		if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
> +		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
> +		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> +		    zswap_present_test(entry))

This should really be abstracted into a reasonably-named helper that can live in
swap code.

-- 
Cheers,

David

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Hi David,

On 6/24/26 16:58, David Hildenbrand (Arm) wrote:
> On 6/24/26 09:55, Alexandre Ghiti wrote:
>> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
>> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>>
>> zswap is the same kind of in-memory, synchronous backend as zram, not a
>> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
>> swapin_readahead().
>>
>> Here are the results from bypassing readahead for zswap too: it was
>> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
>> off, on Sapphire Rapids and 3 iterations.
>>
>>    768M memcg (sustained swap thrash):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          405.0       341.7    -15.6%
>>      zswap-in (GB)            79.5        53.0     -33%
>>      zswap-out (GB)          144.8       115.6     -20%
>>      swap readahead (pages)  6.79M       0.45M     -93%
>>      swap_ra hit (%)          72.1        89.9     +18pp
>>
>>    1G memcg (light pressure, build not memory-bound):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          177.7       176.0    ~same (no regression)
>>      zswap-in (GB)            10.2         7.5     -26%
>>      zswap-out (GB)           27.7        25.1      -9%
>>      swap readahead (pages)  1.07M       0.08M     -93%
>>      swap_ra hit (%)          68.6        87.2     +19pp
>>
>> The gain is from no longer prefetching pages that are pointless for an
>> in-memory backend: readahead inflates anon residency and thrashes the
>> page cache (file pages get evicted and re-read), lengthens each fault by
>> synchronously (de)compressing a cluster of neighbours, and adds
>> compression traffic when those extra pages are reclaimed.
>>
>> Bypassing swap readahead for zswap therefore makes sense.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
> [...]
>
>>   #endif /* _LINUX_ZSWAP_H */
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ff338c2abe92..5aa1ea9eb48a 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>   	if (folio)
>>   		swap_update_readahead(folio, vma, vmf->address);
>>   	if (!folio) {
>> -		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
>> -		if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
>> +		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
>> +		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
>> +		    zswap_present_test(entry))
> This should really be abstracted into a reasonably-named helper that can live in
> swap code.
>

Makes sense, I'll come up with something.

Thanks,

Alex

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Yosry Ahmed]

On Wed, Jun 24, 2026 at 12:57 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>
> zswap is the same kind of in-memory, synchronous backend as zram, not a
> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
> swapin_readahead().
>
> Here are the results from bypassing readahead for zswap too: it was
> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
> off, on Sapphire Rapids and 3 iterations.
>
>   768M memcg (sustained swap thrash):
>     metric                 mm-new    + bypass    delta
>     build time (s)          405.0       341.7    -15.6%
>     zswap-in (GB)            79.5        53.0     -33%
>     zswap-out (GB)          144.8       115.6     -20%
>     swap readahead (pages)  6.79M       0.45M     -93%
>     swap_ra hit (%)          72.1        89.9     +18pp
>
>   1G memcg (light pressure, build not memory-bound):
>     metric                 mm-new    + bypass    delta
>     build time (s)          177.7       176.0    ~same (no regression)
>     zswap-in (GB)            10.2         7.5     -26%
>     zswap-out (GB)           27.7        25.1      -9%
>     swap readahead (pages)  1.07M       0.08M     -93%
>     swap_ra hit (%)          68.6        87.2     +19pp
>
> The gain is from no longer prefetching pages that are pointless for an
> in-memory backend: readahead inflates anon residency and thrashes the
> page cache (file pages get evicted and re-read), lengthens each fault by
> synchronously (de)compressing a cluster of neighbours, and adds
> compression traffic when those extra pages are reclaimed.
>
> Bypassing swap readahead for zswap therefore makes sense.
>
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
>
> - This bypass originally comes from Usama's series that implements
>   large folio zswapin: while working on improving this series, I noticed
>   the gains I got only came from the bypass of readahead.
>
>  include/linux/zswap.h |  6 ++++++
>  mm/memory.c           |  5 +++--
>  mm/zswap.c            | 11 +++++++++++
>  3 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 30c193a1207e..b6f0e6198b6f 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>  void zswap_folio_swapin(struct folio *folio);
>  bool zswap_is_enabled(void);
>  bool zswap_never_enabled(void);
> +bool zswap_present_test(swp_entry_t swp);
>  #else
>
>  struct zswap_lruvec_state {};
> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>         return true;
>  }
>
> +static inline bool zswap_present_test(swp_entry_t swp)
> +{
> +       return false;
> +}
> +
>  #endif
>
>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index ff338c2abe92..5aa1ea9eb48a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         if (folio)
>                 swap_update_readahead(folio, vma, vmf->address);
>         if (!folio) {
> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> +                   zswap_present_test(entry))

This assumes that if the swap entry is in zswap, then the remaining
entries (covered by the readahead window) will also be in zswap,
right? While not very likely, it's possible that the remaining entries
not in zswap but on disk, right?

>                         folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
>                                             thp_swapin_suitable_orders(vmf) | BIT(0),
>                                             vmf, NULL, 0);
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 761cd699e0a3..5b85b4d17647 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>                 >> ZSWAP_ADDRESS_SPACE_SHIFT];
>  }
>
> +/**
> + * zswap_present_test - check if a swap entry is currently backed by zswap
> + * @swp: the swap entry to test
> + *
> + * Return: true if @swp has a zswap entry, false otherwise.
> + */
> +bool zswap_present_test(swp_entry_t swp)

zswap_is_present()?

> +{
> +       return xa_load(swap_zswap_tree(swp), swp_offset(swp));
> +}
> +
>  #define zswap_pool_debug(msg, p)                       \
>         pr_debug("%s pool %s\n", msg, (p)->tfm_name)
>
> --
> 2.54.0
>
>

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Hi Yosry,

On 6/24/26 20:01, Yosry Ahmed wrote:
> On Wed, Jun 24, 2026 at 12:57 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
>> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>>
>> zswap is the same kind of in-memory, synchronous backend as zram, not a
>> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
>> swapin_readahead().
>>
>> Here are the results from bypassing readahead for zswap too: it was
>> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
>> off, on Sapphire Rapids and 3 iterations.
>>
>>    768M memcg (sustained swap thrash):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          405.0       341.7    -15.6%
>>      zswap-in (GB)            79.5        53.0     -33%
>>      zswap-out (GB)          144.8       115.6     -20%
>>      swap readahead (pages)  6.79M       0.45M     -93%
>>      swap_ra hit (%)          72.1        89.9     +18pp
>>
>>    1G memcg (light pressure, build not memory-bound):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          177.7       176.0    ~same (no regression)
>>      zswap-in (GB)            10.2         7.5     -26%
>>      zswap-out (GB)           27.7        25.1      -9%
>>      swap readahead (pages)  1.07M       0.08M     -93%
>>      swap_ra hit (%)          68.6        87.2     +19pp
>>
>> The gain is from no longer prefetching pages that are pointless for an
>> in-memory backend: readahead inflates anon residency and thrashes the
>> page cache (file pages get evicted and re-read), lengthens each fault by
>> synchronously (de)compressing a cluster of neighbours, and adds
>> compression traffic when those extra pages are reclaimed.
>>
>> Bypassing swap readahead for zswap therefore makes sense.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>
>> - This bypass originally comes from Usama's series that implements
>>    large folio zswapin: while working on improving this series, I noticed
>>    the gains I got only came from the bypass of readahead.
>>
>>   include/linux/zswap.h |  6 ++++++
>>   mm/memory.c           |  5 +++--
>>   mm/zswap.c            | 11 +++++++++++
>>   3 files changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
>> index 30c193a1207e..b6f0e6198b6f 100644
>> --- a/include/linux/zswap.h
>> +++ b/include/linux/zswap.h
>> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>>   void zswap_folio_swapin(struct folio *folio);
>>   bool zswap_is_enabled(void);
>>   bool zswap_never_enabled(void);
>> +bool zswap_present_test(swp_entry_t swp);
>>   #else
>>
>>   struct zswap_lruvec_state {};
>> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>>          return true;
>>   }
>>
>> +static inline bool zswap_present_test(swp_entry_t swp)
>> +{
>> +       return false;
>> +}
>> +
>>   #endif
>>
>>   #endif /* _LINUX_ZSWAP_H */
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ff338c2abe92..5aa1ea9eb48a 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>          if (folio)
>>                  swap_update_readahead(folio, vma, vmf->address);
>>          if (!folio) {
>> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
>> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
>> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
>> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
>> +                   zswap_present_test(entry))
> This assumes that if the swap entry is in zswap, then the remaining
> entries (covered by the readahead window) will also be in zswap,
> right? While not very likely, it's possible that the remaining entries
> not in zswap but on disk, right?


Yes, I assumed locality here.

Indeed, it would be interesting to keep the readahead but only actually 
readahead swap disk entries. I don't know how this will affect the 
readahead window (it is computed from the number of PG_readahead hits 
iirc) but I can give it a try.


>
>>                          folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
>>                                              thp_swapin_suitable_orders(vmf) | BIT(0),
>>                                              vmf, NULL, 0);
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 761cd699e0a3..5b85b4d17647 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>>                  >> ZSWAP_ADDRESS_SPACE_SHIFT];
>>   }
>>
>> +/**
>> + * zswap_present_test - check if a swap entry is currently backed by zswap
>> + * @swp: the swap entry to test
>> + *
>> + * Return: true if @swp has a zswap entry, false otherwise.
>> + */
>> +bool zswap_present_test(swp_entry_t swp)
> zswap_is_present()?


Agree, the naming is not perfect, I'll change that to either your 
proposal or zswap_is_entry_present() (or something else), but I'll 
definitely change that.

Thanks,

Alex


>
>> +{
>> +       return xa_load(swap_zswap_tree(swp), swp_offset(swp));
>> +}
>> +
>>   #define zswap_pool_debug(msg, p)                       \
>>          pr_debug("%s pool %s\n", msg, (p)->tfm_name)
>>
>> --
>> 2.54.0
>>
>>

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Barry Song]

On Wed, Jun 24, 2026 at 3:57 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>
> zswap is the same kind of in-memory, synchronous backend as zram, not a
> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
> swapin_readahead().
>
> Here are the results from bypassing readahead for zswap too: it was
> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
> off, on Sapphire Rapids and 3 iterations.
>
>   768M memcg (sustained swap thrash):
>     metric                 mm-new    + bypass    delta
>     build time (s)          405.0       341.7    -15.6%
>     zswap-in (GB)            79.5        53.0     -33%
>     zswap-out (GB)          144.8       115.6     -20%
>     swap readahead (pages)  6.79M       0.45M     -93%
>     swap_ra hit (%)          72.1        89.9     +18pp
>
>   1G memcg (light pressure, build not memory-bound):
>     metric                 mm-new    + bypass    delta
>     build time (s)          177.7       176.0    ~same (no regression)
>     zswap-in (GB)            10.2         7.5     -26%
>     zswap-out (GB)           27.7        25.1      -9%
>     swap readahead (pages)  1.07M       0.08M     -93%
>     swap_ra hit (%)          68.6        87.2     +19pp
>
> The gain is from no longer prefetching pages that are pointless for an
> in-memory backend: readahead inflates anon residency and thrashes the
> page cache (file pages get evicted and re-read), lengthens each fault by
> synchronously (de)compressing a cluster of neighbours, and adds
> compression traffic when those extra pages are reclaimed.
>
> Bypassing swap readahead for zswap therefore makes sense.
>
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
>
> - This bypass originally comes from Usama's series that implements
>   large folio zswapin: while working on improving this series, I noticed
>   the gains I got only came from the bypass of readahead.
>
>  include/linux/zswap.h |  6 ++++++
>  mm/memory.c           |  5 +++--
>  mm/zswap.c            | 11 +++++++++++
>  3 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 30c193a1207e..b6f0e6198b6f 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>  void zswap_folio_swapin(struct folio *folio);
>  bool zswap_is_enabled(void);
>  bool zswap_never_enabled(void);
> +bool zswap_present_test(swp_entry_t swp);
>  #else
>
>  struct zswap_lruvec_state {};
> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>         return true;
>  }
>
> +static inline bool zswap_present_test(swp_entry_t swp)
> +{
> +       return false;
> +}
> +
>  #endif
>
>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index ff338c2abe92..5aa1ea9eb48a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         if (folio)
>                 swap_update_readahead(folio, vma, vmf->address);
>         if (!folio) {
> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> +                   zswap_present_test(entry))
>                         folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
>                                             thp_swapin_suitable_orders(vmf) | BIT(0),
>                                             vmf, NULL, 0);

Basically, I have been seeing the same issue recently. If the
readahead swap entries are also in zswap, we end up doing the
decompression during one page fault, but then need another page fault
to fetch the page from the swap cache and install the mapping. In that
case, readahead may not be beneficial.

On the other hand, if the readahead swap entries are not in zswap, the
situation is different.

For example, suppose we fault on the swap entry for address 1 MB and
readahead brings in the entry for 1 MB + 4 KB. If both entries are in
zswap, readahead does not seem like a good trade-off. However, if the
1 MB + 4 KB entry is not in zswap and would otherwise require storage
I/O, then readahead can be beneficial.

So I implemented a rather ugly fault_around-like mechanism in
do_swap_page(). At least with page-cluster == 1, I am seeing a
performance improvement, as the readahead folios can be mapped
directly and do not require a second page fault.

It is admittedly quite ugly and is only meant as a proof of concept :-)

Subject: [PATCH PoC] mm: enable do_swap_page fault_around

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/memory.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index c00a31a6d1d0..1db79f45a575 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4736,6 +4736,100 @@ static void check_swap_exclusive(struct folio
*folio, swp_entry_t entry,
         } while (--nr_pages);
 }

+static void do_swap_map_around(struct vm_fault *vmf, struct
swap_info_struct *si)
+{
+        struct vm_area_struct *vma = vmf->vma;
+        int nr_around = 1 << page_cluster;
+        unsigned long start = max3(vma->vm_start, vmf->address -
(nr_around - 1) * PAGE_SIZE,
+                        vmf->address & PMD_MASK);
+        unsigned long end = min3(vma->vm_end, vmf->address +
nr_around * PAGE_SIZE,
+                        (vmf->address & PMD_MASK) + PMD_SIZE);
+        unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
+        unsigned long delta_pages = (vmf->address - start) >> PAGE_SHIFT;
+        pte_t *ptep = vmf->pte - delta_pages;
+
+        for (int i = 0; i < nr_pages; i++, ptep++) {
+                unsigned long address = start + (i << PAGE_SHIFT);
+                rmap_t rmap_flags = RMAP_NONE;
+                pte_t orig_pte, pte;
+                struct folio *folio;
+                struct page *page;
+                softleaf_t entry;
+                bool exclusive;
+
+                if (ptep == vmf->pte)
+                        continue;
+                orig_pte = ptep_get(ptep);
+                exclusive = pte_swp_exclusive(orig_pte);
+                if (!exclusive)
+                        continue;
+                entry = softleaf_from_pte(orig_pte);
+                if (!softleaf_is_swap(entry))
+                        continue;
+                folio = swap_cache_get_folio(entry);
+                if (!folio)
+                        continue;
+                if (unlikely(!folio_matches_swap_entry(folio, entry)))
+                        goto skip;
+                if (folio_test_locked(folio))
+                        goto skip;
+                if (!folio_test_uptodate(folio))
+                        goto skip;
+                if (!folio_trylock(folio))
+                        goto skip;
+                if (folio_test_ksm(folio) || folio_test_large(folio) ||
+                        !folio_test_uptodate(folio))
+                        goto unlock;
+                if (exclusive && folio_test_writeback(folio) &&
+                                data_race(si->flags & SWP_STABLE_WRITES))
+                        exclusive = false;
+
+                arch_swap_restore(folio_swap(entry, folio), folio);
+
+                page = folio_page(folio, 0);
+                add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
+                add_mm_counter(vma->vm_mm, MM_SWAPENTS, -1);
+                pte = mk_pte(page, vma->vm_page_prot);
+                if (pte_swp_soft_dirty(orig_pte))
+                        pte = pte_mksoft_dirty(pte);
+                if (pte_swp_uffd_wp(orig_pte))
+                        pte = pte_mkuffd_wp(pte);
+
+                if (exclusive) {
+                        if ((vma->vm_flags & VM_WRITE) &&
!userfaultfd_pte_wp(vma, pte) &&
+                                        !pte_needs_soft_dirty_wp(vma, pte)) {
+                                pte = pte_mkwrite(pte, vma);
+                        }
+                        rmap_flags |= RMAP_EXCLUSIVE;
+                }
+                flush_icache_pages(vma, page, 1);
+
+                if (!folio_test_anon(folio)) {
+                        folio_add_new_anon_rmap(folio, vma, address,
rmap_flags);
+                        folio_put_swap(folio, NULL);
+                } else {
+                        folio_add_anon_rmap_ptes(folio, page, 1, vma, address,
+                                        rmap_flags);
+                        folio_put_swap(folio, page);
+                }
+
+                set_ptes(vma->vm_mm, address, ptep, pte, 1);
+                arch_do_swap_page_nr(vma->vm_mm, vma, address,
+                                pte, pte, 1);
+
+                if (should_try_to_free_swap(si, folio, vma, 1, vmf->flags))
+                        folio_free_swap(folio);
+                folio_unlock(folio);
+                swap_update_readahead(folio, vma, address);
+                update_mmu_cache_range(vmf, vma, address, ptep, 1);
+                continue;
+unlock:
+                folio_unlock(folio);
+skip:
+                folio_put(folio);
+        };
+}
+
 /*
  * We enter with either the VMA lock or the mmap_lock held (see
  * FAULT_FLAG_VMA_LOCK), and pte mapped but not yet locked.
@@ -5121,6 +5215,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)

         /* No need to invalidate - it was non-present before */
         update_mmu_cache_range(vmf, vma, address, ptep, nr_pages);
+        do_swap_map_around(vmf, si);
 unlock:
         if (vmf->pte)
                 pte_unmap_unlock(vmf->pte, vmf->ptl);
-- 
2.34.1

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Hi Barry,

On 6/24/26 21:24, Barry Song wrote:
> On Wed, Jun 24, 2026 at 3:57 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
>> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
>> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>>
>> zswap is the same kind of in-memory, synchronous backend as zram, not a
>> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
>> swapin_readahead().
>>
>> Here are the results from bypassing readahead for zswap too: it was
>> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
>> off, on Sapphire Rapids and 3 iterations.
>>
>>    768M memcg (sustained swap thrash):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          405.0       341.7    -15.6%
>>      zswap-in (GB)            79.5        53.0     -33%
>>      zswap-out (GB)          144.8       115.6     -20%
>>      swap readahead (pages)  6.79M       0.45M     -93%
>>      swap_ra hit (%)          72.1        89.9     +18pp
>>
>>    1G memcg (light pressure, build not memory-bound):
>>      metric                 mm-new    + bypass    delta
>>      build time (s)          177.7       176.0    ~same (no regression)
>>      zswap-in (GB)            10.2         7.5     -26%
>>      zswap-out (GB)           27.7        25.1      -9%
>>      swap readahead (pages)  1.07M       0.08M     -93%
>>      swap_ra hit (%)          68.6        87.2     +19pp
>>
>> The gain is from no longer prefetching pages that are pointless for an
>> in-memory backend: readahead inflates anon residency and thrashes the
>> page cache (file pages get evicted and re-read), lengthens each fault by
>> synchronously (de)compressing a cluster of neighbours, and adds
>> compression traffic when those extra pages are reclaimed.
>>
>> Bypassing swap readahead for zswap therefore makes sense.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>
>> - This bypass originally comes from Usama's series that implements
>>    large folio zswapin: while working on improving this series, I noticed
>>    the gains I got only came from the bypass of readahead.
>>
>>   include/linux/zswap.h |  6 ++++++
>>   mm/memory.c           |  5 +++--
>>   mm/zswap.c            | 11 +++++++++++
>>   3 files changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
>> index 30c193a1207e..b6f0e6198b6f 100644
>> --- a/include/linux/zswap.h
>> +++ b/include/linux/zswap.h
>> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>>   void zswap_folio_swapin(struct folio *folio);
>>   bool zswap_is_enabled(void);
>>   bool zswap_never_enabled(void);
>> +bool zswap_present_test(swp_entry_t swp);
>>   #else
>>
>>   struct zswap_lruvec_state {};
>> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>>          return true;
>>   }
>>
>> +static inline bool zswap_present_test(swp_entry_t swp)
>> +{
>> +       return false;
>> +}
>> +
>>   #endif
>>
>>   #endif /* _LINUX_ZSWAP_H */
>> diff --git a/mm/memory.c b/mm/memory.c
>> index ff338c2abe92..5aa1ea9eb48a 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>          if (folio)
>>                  swap_update_readahead(folio, vma, vmf->address);
>>          if (!folio) {
>> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
>> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
>> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
>> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
>> +                   zswap_present_test(entry))
>>                          folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
>>                                              thp_swapin_suitable_orders(vmf) | BIT(0),
>>                                              vmf, NULL, 0);
> Basically, I have been seeing the same issue recently. If the
> readahead swap entries are also in zswap, we end up doing the
> decompression during one page fault, but then need another page fault
> to fetch the page from the swap cache and install the mapping. In that
> case, readahead may not be beneficial.


Oh I had not noticed that, indeed since zswap readahead is synchronous, 
we can clearly avoid the second page fault!


>
> On the other hand, if the readahead swap entries are not in zswap, the
> situation is different.
>
> For example, suppose we fault on the swap entry for address 1 MB and
> readahead brings in the entry for 1 MB + 4 KB. If both entries are in
> zswap, readahead does not seem like a good trade-off. However, if the
> 1 MB + 4 KB entry is not in zswap and would otherwise require storage
> I/O, then readahead can be beneficial.


Yosry made the same comment, I'll explore this.


>
> So I implemented a rather ugly fault_around-like mechanism in
> do_swap_page(). At least with page-cluster == 1, I am seeing a
> performance improvement, as the readahead folios can be mapped
> directly and do not require a second page fault.


IIUC the code below, you wait for the end of the page fault to try and 
map a folio that would have been readahead right? I guess you do that at 
the end in the hope that the io has finished by then?

Maybe we can do that synchronously for zswap since the readahead is 
synchronous?  And for the readahead pages that require io, wouldn't it 
be possible to do it in the end of io callback instead?

Thanks for your comments,

Alex


>
> It is admittedly quite ugly and is only meant as a proof of concept :-)
>
> Subject: [PATCH PoC] mm: enable do_swap_page fault_around
>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>   mm/memory.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 95 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index c00a31a6d1d0..1db79f45a575 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4736,6 +4736,100 @@ static void check_swap_exclusive(struct folio
> *folio, swp_entry_t entry,
>           } while (--nr_pages);
>   }
>
> +static void do_swap_map_around(struct vm_fault *vmf, struct
> swap_info_struct *si)
> +{
> +        struct vm_area_struct *vma = vmf->vma;
> +        int nr_around = 1 << page_cluster;
> +        unsigned long start = max3(vma->vm_start, vmf->address -
> (nr_around - 1) * PAGE_SIZE,
> +                        vmf->address & PMD_MASK);
> +        unsigned long end = min3(vma->vm_end, vmf->address +
> nr_around * PAGE_SIZE,
> +                        (vmf->address & PMD_MASK) + PMD_SIZE);
> +        unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> +        unsigned long delta_pages = (vmf->address - start) >> PAGE_SHIFT;
> +        pte_t *ptep = vmf->pte - delta_pages;
> +
> +        for (int i = 0; i < nr_pages; i++, ptep++) {
> +                unsigned long address = start + (i << PAGE_SHIFT);
> +                rmap_t rmap_flags = RMAP_NONE;
> +                pte_t orig_pte, pte;
> +                struct folio *folio;
> +                struct page *page;
> +                softleaf_t entry;
> +                bool exclusive;
> +
> +                if (ptep == vmf->pte)
> +                        continue;
> +                orig_pte = ptep_get(ptep);
> +                exclusive = pte_swp_exclusive(orig_pte);
> +                if (!exclusive)
> +                        continue;
> +                entry = softleaf_from_pte(orig_pte);
> +                if (!softleaf_is_swap(entry))
> +                        continue;
> +                folio = swap_cache_get_folio(entry);
> +                if (!folio)
> +                        continue;
> +                if (unlikely(!folio_matches_swap_entry(folio, entry)))
> +                        goto skip;
> +                if (folio_test_locked(folio))
> +                        goto skip;
> +                if (!folio_test_uptodate(folio))
> +                        goto skip;
> +                if (!folio_trylock(folio))
> +                        goto skip;
> +                if (folio_test_ksm(folio) || folio_test_large(folio) ||
> +                        !folio_test_uptodate(folio))
> +                        goto unlock;
> +                if (exclusive && folio_test_writeback(folio) &&
> +                                data_race(si->flags & SWP_STABLE_WRITES))
> +                        exclusive = false;
> +
> +                arch_swap_restore(folio_swap(entry, folio), folio);
> +
> +                page = folio_page(folio, 0);
> +                add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> +                add_mm_counter(vma->vm_mm, MM_SWAPENTS, -1);
> +                pte = mk_pte(page, vma->vm_page_prot);
> +                if (pte_swp_soft_dirty(orig_pte))
> +                        pte = pte_mksoft_dirty(pte);
> +                if (pte_swp_uffd_wp(orig_pte))
> +                        pte = pte_mkuffd_wp(pte);
> +
> +                if (exclusive) {
> +                        if ((vma->vm_flags & VM_WRITE) &&
> !userfaultfd_pte_wp(vma, pte) &&
> +                                        !pte_needs_soft_dirty_wp(vma, pte)) {
> +                                pte = pte_mkwrite(pte, vma);
> +                        }
> +                        rmap_flags |= RMAP_EXCLUSIVE;
> +                }
> +                flush_icache_pages(vma, page, 1);
> +
> +                if (!folio_test_anon(folio)) {
> +                        folio_add_new_anon_rmap(folio, vma, address,
> rmap_flags);
> +                        folio_put_swap(folio, NULL);
> +                } else {
> +                        folio_add_anon_rmap_ptes(folio, page, 1, vma, address,
> +                                        rmap_flags);
> +                        folio_put_swap(folio, page);
> +                }
> +
> +                set_ptes(vma->vm_mm, address, ptep, pte, 1);
> +                arch_do_swap_page_nr(vma->vm_mm, vma, address,
> +                                pte, pte, 1);
> +
> +                if (should_try_to_free_swap(si, folio, vma, 1, vmf->flags))
> +                        folio_free_swap(folio);
> +                folio_unlock(folio);
> +                swap_update_readahead(folio, vma, address);
> +                update_mmu_cache_range(vmf, vma, address, ptep, 1);
> +                continue;
> +unlock:
> +                folio_unlock(folio);
> +skip:
> +                folio_put(folio);
> +        };
> +}
> +
>   /*
>    * We enter with either the VMA lock or the mmap_lock held (see
>    * FAULT_FLAG_VMA_LOCK), and pte mapped but not yet locked.
> @@ -5121,6 +5215,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>
>           /* No need to invalidate - it was non-present before */
>           update_mmu_cache_range(vmf, vma, address, ptep, nr_pages);
> +        do_swap_map_around(vmf, si);
>   unlock:
>           if (vmf->pte)
>                   pte_unmap_unlock(vmf->pte, vmf->ptl);

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Nhat Pham]

On Wed, Jun 24, 2026 at 12:24 PM Barry Song <baohua@kernel.org> wrote:
>
> Basically, I have been seeing the same issue recently. If the
> readahead swap entries are also in zswap, we end up doing the
> decompression during one page fault, but then need another page fault
> to fetch the page from the swap cache and install the mapping. In that
> case, readahead may not be beneficial.

You're playing with zswap? Nice :)

It does feel like we're leaving some obvious wins on the table with
zswap here, so any ideas are much appreciated :)

>
> On the other hand, if the readahead swap entries are not in zswap, the
> situation is different.
>
> For example, suppose we fault on the swap entry for address 1 MB and
> readahead brings in the entry for 1 MB + 4 KB. If both entries are in
> zswap, readahead does not seem like a good trade-off. However, if the
> 1 MB + 4 KB entry is not in zswap and would otherwise require storage
> I/O, then readahead can be beneficial.

Yeah I can see the edge case you and Yosry brought up here. As we move
towards supporting multiple swap backends in the same system, that can
certainly happen. I'm hoping that locality will bail us out - pages
close together in virtual address space hopefully have similar
lifetime, access temperature, compressibility, and will go to the same
swap backend, etc. - but who knows :)

What if we still follow the readahead logic, but at each slot in the
readahead window, we check if the entry is owned by zswap or is of a
sync-io devices, in which case we skip the readahead...?

That way, we will still submit the IO work, but spare the ones that
require synchronous decompression work, which might affect latency...?

>
> So I implemented a rather ugly fault_around-like mechanism in
> do_swap_page(). At least with page-cluster == 1, I am seeing a
> performance improvement, as the readahead folios can be mapped
> directly and do not require a second page fault.

Hmm so what your proposal buys us is, if we're already doing readahead
(synchronously), might as well install the pages in the window into
the page tables? :)

Could you explain to me how does this improve performance? If the
pages are needed, we are transferring some work from future page
faults to the current page fault. If the pages are not needed, then we
are just wasting cycles right?

>
> It is admittedly quite ugly and is only meant as a proof of concept :-)

Dw, arguably it's easier to reason about with everything open-coded ;)

Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Barry Song]

On Fri, Jun 26, 2026 at 12:39 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> On Wed, Jun 24, 2026 at 12:24 PM Barry Song <baohua@kernel.org> wrote:
> >
> > Basically, I have been seeing the same issue recently. If the
> > readahead swap entries are also in zswap, we end up doing the
> > decompression during one page fault, but then need another page fault
> > to fetch the page from the swap cache and install the mapping. In that
> > case, readahead may not be beneficial.
>
> You're playing with zswap? Nice :)

Hi Nhat,

Sorry for the late reply. I've been quite busy recently.
I used to work on zswap when I was in the server industry. These days, I
still play with it from time to time, mostly for fun.

>
> It does feel like we're leaving some obvious wins on the table with
> zswap here, so any ideas are much appreciated :)

For my part, I'm not against either zram or zswap. They both play
important roles in their respective domains. :-)

>
> >
> > On the other hand, if the readahead swap entries are not in zswap, the
> > situation is different.
> >
> > For example, suppose we fault on the swap entry for address 1 MB and
> > readahead brings in the entry for 1 MB + 4 KB. If both entries are in
> > zswap, readahead does not seem like a good trade-off. However, if the
> > 1 MB + 4 KB entry is not in zswap and would otherwise require storage
> > I/O, then readahead can be beneficial.
>
> Yeah I can see the edge case you and Yosry brought up here. As we move
> towards supporting multiple swap backends in the same system, that can
> certainly happen. I'm hoping that locality will bail us out - pages
> close together in virtual address space hopefully have similar
> lifetime, access temperature, compressibility, and will go to the same
> swap backend, etc. - but who knows :)
>
> What if we still follow the readahead logic, but at each slot in the
> readahead window, we check if the entry is owned by zswap or is of a
> sync-io devices, in which case we skip the readahead...?

That sounds fine to me. I'm just not sure how complex the code would
be. Would the readahead path need to check whether folios are in zswap
or not? That sounds like a pain.

>
> That way, we will still submit the IO work, but spare the ones that
> require synchronous decompression work, which might affect latency...?

Yes, but it seems like it could get quite complex. So I'm fine with
either the current approach in this patch, or simply disabling
readahead once we're sure we're using zswap, without even checking
whether the current swap-in is coming from zswap.

>
> >
> > So I implemented a rather ugly fault_around-like mechanism in
> > do_swap_page(). At least with page-cluster == 1, I am seeing a
> > performance improvement, as the readahead folios can be mapped
> > directly and do not require a second page fault.
>
> Hmm so what your proposal buys us is, if we're already doing readahead
> (synchronously), might as well install the pages in the window into
> the page tables? :)

Yes, because it avoids another page fault. Handling two pages in a
single page fault is faster than handling two separate page faults,
each faulting in one page. Since we've already read the data out
synchronously, we don't want to fault on it again. The downside is
that it increases the complexity of do_swap_page().

I'm not even trying to sell my approach—I did it just for fun.
do_swap_page() is already quite complex, and my fault-around
implementation only makes it more complicated. So I don't really want
to push for it to be merged upstream. I think it will become much more
attractive once we have hardware-assisted decompression in
do_swap_page(), where we no longer spend CPU cycles on
decompression.

>
> Could you explain to me how does this improve performance? If the
> pages are needed, we are transferring some work from future page
> faults to the current page fault. If the pages are not needed, then we
> are just wasting cycles right?

Yep. I'm only seeing improvements with page-cluster=1, which uses a
small readahead window. For page-cluster > 2, I'm seeing performance
regressions on workloads such as kernel builds. On the other hand, a
larger page-cluster provides a bigger benefit for sequential swap-ins.
So it's not a universal win.

>
> >
> > It is admittedly quite ugly and is only meant as a proof of concept :-)
>
> Dw, arguably it's easier to reason about with everything open-coded ;)

Thanks for your kind words.

Best Regards
Barry