Re: [PATCH v5 5/9] mm: swap: skip slot cache on freeing for mTHP

[Barry Song <baohua@kernel.org>]

On Sat, Aug 3, 2024 at 9:11 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Jul 31, 2024 at 6:49 PM <chrisl@kernel.org> wrote:
> >
> > From: Kairui Song <kasong@tencent.com>
> >
> > Currently when we are freeing mTHP folios from swap cache, we free
> > then one by one and put each entry into swap slot cache. 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 <kasong@tencent.com>
>
> Acked-by: Barry Song <baohua@kernel.org>
>
> 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 <v-songbaohua@oppo.com>
> 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 <v-songbaohua@oppo.com>
> ---
>  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;
> +               }

Sorry, what I actually meant was that the two gotos are reversed.

                if (batched_free) {
                        if (any_only_cache)
                                goto reclaim;

                        spin_lock(&si->lock);
                        swap_entry_range_free(si, entry, nr);
                        spin_unlock(&si->lock);
                        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
>