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Slot > cache is designed to reduce the overhead by batching the freeing, > but mTHP swap entries are already continuous so they can be batch > freed without it already, it saves litle overhead, or even increase > overhead for larger mTHP. > > What's more, mTHP entries could stay in swap cache for a while. > Contiguous swap entry is an rather rare resource so releasing them > directly can help improve mTHP allocation success rate when under > pressure. > > Signed-off-by: Kairui Song Acked-by: Barry Song I believe this is the right direction to take. Currently, entries are released one by one, even when they are contiguous in the swap file(those nr_pages entries are definitely in the same cluster and same si), leading to numerous lock and unlock operations. This approach provides batched support. free_swap_and_cache_nr() has the same issue, so I drafted a patch based on your code. I wonder if you can also help test and review before I send it officially: >From 4bed5c08bc0f7769ee2849812acdad70c4e32ead Mon Sep 17 00:00:00 2001 From: Barry Song Date: Sat, 3 Aug 2024 20:21:14 +1200 Subject: [PATCH RFC] mm: attempt to batch free swap entries for zap_pte_range() Zhiguo reported that swap release could be a serious bottleneck during process exits[1]. With mTHP, we have the opportunity to batch free swaps. Thanks to the work of Chris and Kairui[2], I was able to achieve this optimization with minimal code changes by building on their efforts. [1] https://lore.kernel.org/linux-mm/20240731133318.527-1-justinjiang@vivo.com/ [2] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@kernel.org/ Signed-off-by: Barry Song --- mm/swapfile.c | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) diff --git a/mm/swapfile.c b/mm/swapfile.c index ea023fc25d08..9def6dba8d26 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -156,6 +156,25 @@ static bool swap_is_has_cache(struct swap_info_struct *si, return true; } +static bool swap_is_last_map(struct swap_info_struct *si, + unsigned long offset, int nr_pages, + bool *any_only_cache) +{ + unsigned char *map = si->swap_map + offset; + unsigned char *map_end = map + nr_pages; + bool cached = false; + + do { + if ((*map & ~SWAP_HAS_CACHE) != 1) + return false; + if (*map & SWAP_HAS_CACHE) + cached = true; + } while (++map < map_end); + + *any_only_cache = cached; + return true; +} + /* * returns number of pages in the folio that backs the swap entry. If positive, * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no @@ -1808,6 +1827,29 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr) if (WARN_ON(end_offset > si->max)) goto out; + if (nr > 1) { + struct swap_cluster_info *ci; + bool batched_free; + int i; + + ci = lock_cluster_or_swap_info(si, start_offset); + if ((batched_free = swap_is_last_map(si, start_offset, nr, &any_only_cache))) { + for (i = 0; i < nr; i++) + WRITE_ONCE(si->swap_map[start_offset + i], SWAP_HAS_CACHE); + } + unlock_cluster_or_swap_info(si, ci); + + if (batched_free) { + spin_lock(&si->lock); + pr_err("%s offset:%lx nr:%lx\n", __func__,start_offset, nr); + swap_entry_range_free(si, entry, nr); + spin_unlock(&si->lock); + if (any_only_cache) + goto reclaim; + goto out; + } + } + /* * First free all entries in the range. */ @@ -1828,6 +1870,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr) if (!any_only_cache) goto out; +reclaim: /* * Now go back over the range trying to reclaim the swap cache. This is * more efficient for large folios because we will only try to reclaim -- 2.34.1 > --- >  mm/swapfile.c | 59 ++++++++++++++++++++++++++--------------------------------- >  1 file changed, 26 insertions(+), 33 deletions(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 34e6ea13e8e4..9b63b2262cc2 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -479,20 +479,21 @@ static void inc_cluster_info_page(struct swap_info_struct *p, >  } > >  /* > - * The cluster ci decreases one usage. If the usage counter becomes 0, > + * The cluster ci decreases @nr_pages usage. If the usage counter becomes 0, >   * which means no page in the cluster is in use, we can optionally discard >   * the cluster and add it to free cluster list. >   */ > -static void dec_cluster_info_page(struct swap_info_struct *p, struct swap_cluster_info *ci) > +static void dec_cluster_info_page(struct swap_info_struct *p, > +                                 struct swap_cluster_info *ci, int nr_pages) >  { >         if (!p->cluster_info) >                 return; > > -       VM_BUG_ON(ci->count == 0); > +       VM_BUG_ON(ci->count < nr_pages); >         VM_BUG_ON(cluster_is_free(ci)); >         lockdep_assert_held(&p->lock); >         lockdep_assert_held(&ci->lock); > -       ci->count--; > +       ci->count -= nr_pages; > >         if (!ci->count) { >                 free_cluster(p, ci); > @@ -998,19 +999,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si, >         return n_ret; >  } > > -static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx) > -{ > -       unsigned long offset = idx * SWAPFILE_CLUSTER; > -       struct swap_cluster_info *ci; > - > -       ci = lock_cluster(si, offset); > -       memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER); > -       ci->count = 0; > -       free_cluster(si, ci); > -       unlock_cluster(ci); > -       swap_range_free(si, offset, SWAPFILE_CLUSTER); > -} > - >  int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order) >  { >         int order = swap_entry_order(entry_order); > @@ -1269,21 +1257,28 @@ static unsigned char __swap_entry_free(struct swap_info_struct *p, >         return usage; >  } > > -static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry) > +/* > + * Drop the last HAS_CACHE flag of swap entries, caller have to > + * ensure all entries belong to the same cgroup. > + */ > +static void swap_entry_range_free(struct swap_info_struct *p, swp_entry_t entry, > +                                 unsigned int nr_pages) >  { > -       struct swap_cluster_info *ci; >         unsigned long offset = swp_offset(entry); > -       unsigned char count; > +       unsigned char *map = p->swap_map + offset; > +       unsigned char *map_end = map + nr_pages; > +       struct swap_cluster_info *ci; > >         ci = lock_cluster(p, offset); > -       count = p->swap_map[offset]; > -       VM_BUG_ON(count != SWAP_HAS_CACHE); > -       p->swap_map[offset] = 0; > -       dec_cluster_info_page(p, ci); > +       do { > +               VM_BUG_ON(*map != SWAP_HAS_CACHE); > +               *map = 0; > +       } while (++map < map_end); > +       dec_cluster_info_page(p, ci, nr_pages); >         unlock_cluster(ci); > > -       mem_cgroup_uncharge_swap(entry, 1); > -       swap_range_free(p, offset, 1); > +       mem_cgroup_uncharge_swap(entry, nr_pages); > +       swap_range_free(p, offset, nr_pages); >  } > >  static void cluster_swap_free_nr(struct swap_info_struct *sis, > @@ -1343,7 +1338,6 @@ void swap_free_nr(swp_entry_t entry, int nr_pages) >  void put_swap_folio(struct folio *folio, swp_entry_t entry) >  { >         unsigned long offset = swp_offset(entry); > -       unsigned long idx = offset / SWAPFILE_CLUSTER; >         struct swap_cluster_info *ci; >         struct swap_info_struct *si; >         unsigned char *map; > @@ -1356,19 +1350,18 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry) >                 return; > >         ci = lock_cluster_or_swap_info(si, offset); > -       if (size == SWAPFILE_CLUSTER) { > +       if (size > 1) { >                 map = si->swap_map + offset; > -               for (i = 0; i < SWAPFILE_CLUSTER; i++) { > +               for (i = 0; i < size; i++) { >                         val = map[i]; >                         VM_BUG_ON(!(val & SWAP_HAS_CACHE)); >                         if (val == SWAP_HAS_CACHE) >                                 free_entries++; >                 } > -               if (free_entries == SWAPFILE_CLUSTER) { > +               if (free_entries == size) { >                         unlock_cluster_or_swap_info(si, ci); >                         spin_lock(&si->lock); > -                       mem_cgroup_uncharge_swap(entry, SWAPFILE_CLUSTER); > -                       swap_free_cluster(si, idx); > +                       swap_entry_range_free(si, entry, size); >                         spin_unlock(&si->lock); >                         return; >                 } > @@ -1413,7 +1406,7 @@ void swapcache_free_entries(swp_entry_t *entries, int n) >         for (i = 0; i < n; ++i) { >                 p = swap_info_get_cont(entries[i], prev); >                 if (p) > -                       swap_entry_free(p, entries[i]); > +                       swap_entry_range_free(p, entries[i], 1); >                 prev = p; >         } >         if (p) > > -- > 2.46.0.rc1.232.g9752f9e123-goog > Thanks Barry