Re: [PATCH] fixup: mm/huge_memory.c: introduce folio_split_unmapped

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 11/20/25 10:25, Balbir Singh wrote:
> On 11/20/25 20:09, David Hildenbrand (Red Hat) wrote:
>> On 11/20/25 04:07, Balbir Singh wrote:
>>> Code refactoring of __folio_split() via helper
>>> __folio_freeze_and_split_unmapped() caused a regression with clang-20
>>> with CONFIG_SHMEM=n, the compiler was not able to optimize away the
>>> call to shmem_uncharge() due to changes in nr_shmem_dropped.
>>> Fix this by checking for shmem_mapping() prior to calling
>>> shmem_uncharge(), shmem_mapping() returns false when CONFIG_SHMEM=n.
>>>
>>> smatch also complained about parameter end being used without
>>> initialization, which is a false positive, but keep the tool happy
>>> by sending in initialized parameters. end is initialized to 0.
>>>
>>> Add detailed documentation comments for folio_split_unmapped()
>>>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>>> Cc: Rakie Kim <rakie.kim@sk.com>
>>> Cc: Byungchul Park <byungchul@sk.com>
>>> Cc: Gregory Price <gourry@gourry.net>
>>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
>>> Cc: Alistair Popple <apopple@nvidia.com>
>>> Cc: Oscar Salvador <osalvador@suse.de>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>>> Cc: Nico Pache <npache@redhat.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Dev Jain <dev.jain@arm.com>
>>> Cc: Barry Song <baohua@kernel.org>
>>> Cc: Lyude Paul <lyude@redhat.com>
>>> Cc: Danilo Krummrich <dakr@kernel.org>
>>> Cc: David Airlie <airlied@gmail.com>
>>> Cc: Simona Vetter <simona@ffwll.ch>
>>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>>> Cc: Mika Penttilä <mpenttil@redhat.com>
>>> Cc: Matthew Brost <matthew.brost@intel.com>
>>> Cc: Francois Dugast <francois.dugast@intel.com>
>>>
>>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>>> ---
>>>    mm/huge_memory.c | 32 ++++++++++++++++++++++----------
>>>    1 file changed, 22 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 78a31a476ad3..c4267a0f74df 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3751,6 +3751,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
>>>        int ret = 0;
>>>        struct deferred_split *ds_queue;
>>>    +    VM_WARN_ON_ONCE(!mapping && end != 0);
>>
>> You could drop the "!= 0"
> 
> Ack
> 
> VM_WARN_ONE(!mapping && end);
> 
>>
>>>        /* Prevent deferred_split_scan() touching ->_refcount */
>>>        ds_queue = folio_split_queue_lock(folio);
>>>        if (folio_ref_freeze(folio, 1 + extra_pins)) {
>>> @@ -3919,7 +3920,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>        int nr_shmem_dropped = 0;
>>>        int remap_flags = 0;
>>>        int extra_pins, ret;
>>> -    pgoff_t end;
>>> +    pgoff_t end = 0;
>>>        bool is_hzp;
>>>          VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>>> @@ -4049,7 +4050,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>          local_irq_enable();
>>>    -    if (nr_shmem_dropped)
>>> +    if (mapping && shmem_mapping(mapping) && nr_shmem_dropped)
>>>            shmem_uncharge(mapping->host, nr_shmem_dropped);
>>
>> That looks questionable. We shouldn't add runtime check to handle buildtime things.
>>
>> Likely what you want is instead
>>
>> if (IS_ENABLED(CONFIG_SHMEM) && nr_shmem_dropped)
>>      shmem_uncharge()
>>
> 
> shmem_mapping() returns false for CONFIG_SHMEM=n and shmem_mapping() checks that the mapping
> is indeed for shmem ops before uncharging. Happy to change it if you like,
> your version is more readable
Good point, but the questionable thing is that it looks like nr_shmem_dropped
could be set for non-shmem mappings, when it's really just a compiler thing.

What about handling it through a proper stub so we can keep this calling code simple?

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 5b368f9549d67..e38cb01031200 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -136,11 +136,15 @@ static inline bool shmem_hpage_pmd_enabled(void)
  
  #ifdef CONFIG_SHMEM
  extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
+extern void shmem_uncharge(struct inode *inode, long pages);
  #else
  static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma)
  {
         return 0;
  }
+static inline void shmem_uncharge(struct inode *inode, long pages)
+{
+}
  #endif
  extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
                                                 pgoff_t start, pgoff_t end);
@@ -194,7 +198,6 @@ static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)
  }
  
  extern bool shmem_charge(struct inode *inode, long pages);
-extern void shmem_uncharge(struct inode *inode, long pages);
  
  #ifdef CONFIG_USERFAULTFD
  #ifdef CONFIG_SHMEM


-- 
Cheers

David