Re: [PATCH RFC v2 4/5] mm: swap: introduce swapcache_prepare_nr and swapcache_clear_nr for large folios swap-in

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Feb 28, 2024 at 4:59 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Thu, Feb 29, 2024 at 1:52 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Wed, Feb 28, 2024 at 4:39 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > From: Barry Song <v-songbaohua@oppo.com>
> > >
> > > Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache") supports
> > > one entry only, to support large folio swap-in, we need to handle multiple
> > > swap entries.
> > >
> > > Cc: Kairui Song <kasong@tencent.com>
> > > Cc: "Huang, Ying" <ying.huang@intel.com>
> > > Cc: Yu Zhao <yuzhao@google.com>
> > > Cc: David Hildenbrand <david@redhat.com>
> > > Cc: Chris Li <chrisl@kernel.org>
> > > Cc: Hugh Dickins <hughd@google.com>
> > > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > Cc: Michal Hocko <mhocko@suse.com>
> > > Cc: Minchan Kim <minchan@kernel.org>
> > > Cc: Yosry Ahmed <yosryahmed@google.com>
> > > Cc: Yu Zhao <yuzhao@google.com>
> > > Cc: SeongJae Park <sj@kernel.org>
> > > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > > ---
> > >  include/linux/swap.h |   1 +
> > >  mm/swap.h            |   1 +
> > >  mm/swapfile.c        | 117 ++++++++++++++++++++++++++-----------------
> > >  3 files changed, 72 insertions(+), 47 deletions(-)
> > >
> > > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > > index b3581c976e5f..2691c739d9a4 100644
> > > --- a/include/linux/swap.h
> > > +++ b/include/linux/swap.h
> > > @@ -480,6 +480,7 @@ extern int add_swap_count_continuation(swp_entry_t, gfp_t);
> > >  extern void swap_shmem_alloc(swp_entry_t);
> > >  extern int swap_duplicate(swp_entry_t);
> > >  extern int swapcache_prepare(swp_entry_t);
> > > +extern int swapcache_prepare_nr(swp_entry_t, int nr);
> > >  extern void swap_free(swp_entry_t);
> > >  extern void swap_nr_free(swp_entry_t entry, int nr_pages);
> > >  extern void swapcache_free_entries(swp_entry_t *entries, int n);
> > > diff --git a/mm/swap.h b/mm/swap.h
> > > index fc2f6ade7f80..1cec991efcda 100644
> > > --- a/mm/swap.h
> > > +++ b/mm/swap.h
> > > @@ -42,6 +42,7 @@ void delete_from_swap_cache(struct folio *folio);
> > >  void clear_shadow_from_swap_cache(int type, unsigned long begin,
> > >                                   unsigned long end);
> > >  void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry);
> > > +void swapcache_clear_nr(struct swap_info_struct *si, swp_entry_t entry, int nr);
> > >  struct folio *swap_cache_get_folio(swp_entry_t entry,
> > >                 struct vm_area_struct *vma, unsigned long addr);
> > >  struct folio *filemap_get_incore_folio(struct address_space *mapping,
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index c0c058ee7b69..c8c8b6dbaeda 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -3308,7 +3308,7 @@ void si_swapinfo(struct sysinfo *val)
> > >  }
> > >
> > >  /*
> > > - * Verify that a swap entry is valid and increment its swap map count.
> > > + * Verify that nr swap entries are valid and increment their swap map count.
> > >   *
> > >   * Returns error code in following case.
> > >   * - success -> 0
> > > @@ -3318,66 +3318,73 @@ void si_swapinfo(struct sysinfo *val)
> > >   * - swap-cache reference is requested but the entry is not used. -> ENOENT
> > >   * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
> > >   */
> > > -static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> > > +static int __swap_duplicate_nr(swp_entry_t entry, int nr, unsigned char usage)
> > >  {
> > >         struct swap_info_struct *p;
> > >         struct swap_cluster_info *ci;
> > >         unsigned long offset;
> > > -       unsigned char count;
> > > -       unsigned char has_cache;
> > > -       int err;
> > > +       unsigned char count[SWAPFILE_CLUSTER];
> > > +       unsigned char has_cache[SWAPFILE_CLUSTER];
> >
>
> Hi Yosry,
>
> Thanks for reviewing!
>
> > I am not closely following this series, but a couple of things caught my eyes.
> >
> > Is this reasonable for stack usage?
>
> SWAPFILE_CLUSTER is not huge. typically 512 or 256.

So that's 1K of stack usage out of 16K total on x86. I think this may
be a lot for a single function to use, but perhaps others will
disagree.

>
> #ifdef CONFIG_THP_SWAP
> #define SWAPFILE_CLUSTER        HPAGE_PMD_NR
>
> #define swap_entry_size(size)   (size)
> #else
> #define SWAPFILE_CLUSTER        256
>
> If this is still a concern, I can move it to a bitmap.
>
> >
> > > +       int err, i;
> > >
> > >         p = swp_swap_info(entry);
> > >
> > >         offset = swp_offset(entry);
> > >         ci = lock_cluster_or_swap_info(p, offset);
> > >
> > > -       count = p->swap_map[offset];
> > > -
> > > -       /*
> > > -        * swapin_readahead() doesn't check if a swap entry is valid, so the
> > > -        * swap entry could be SWAP_MAP_BAD. Check here with lock held.
> > > -        */
> > > -       if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
> > > -               err = -ENOENT;
> > > -               goto unlock_out;
> > > -       }
> > > +       for (i = 0; i < nr; i++) {
> > > +               count[i] = p->swap_map[offset + i];
> > >
> > > -       has_cache = count & SWAP_HAS_CACHE;
> > > -       count &= ~SWAP_HAS_CACHE;
> > > -       err = 0;
> > > -
> > > -       if (usage == SWAP_HAS_CACHE) {
> > > -
> > > -               /* set SWAP_HAS_CACHE if there is no cache and entry is used */
> > > -               if (!has_cache && count)
> > > -                       has_cache = SWAP_HAS_CACHE;
> > > -               else if (has_cache)             /* someone else added cache */
> > > -                       err = -EEXIST;
> > > -               else                            /* no users remaining */
> > > +               /*
> > > +                * swapin_readahead() doesn't check if a swap entry is valid, so the
> > > +                * swap entry could be SWAP_MAP_BAD. Check here with lock held.
> > > +                */
> > > +               if (unlikely(swap_count(count[i]) == SWAP_MAP_BAD)) {
> > >                         err = -ENOENT;
> > > +                       goto unlock_out;
> > > +               }
> >
>
>
> > Here we immediately exit if there is an error, but we don't below, we
> > just keep overwriting the error every iteration as far as I can tell.
> > Also, it doesn't seem like we do any cleanups if we hit an error
> > halfway through. Should we undo previously updated swap entries, or am
> > I missing something here?
>
> we are safely immediately exiting because we don't change swap_map
> till we finish all checks. while all checks are done, we write  them by
> WRITE_ONCE(p->swap_map[offset + i], count[i] | has_cache[i]);
> at the end.

I see, but I think we may be overwriting the error from each iteration below?

>
> >
> > >
> > > -       } else if (count || has_cache) {
> > > -
> > > -               if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> > > -                       count += usage;
> > > -               else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> > > -                       err = -EINVAL;
> > > -               else if (swap_count_continued(p, offset, count))
> > > -                       count = COUNT_CONTINUED;
> > > -               else
> > > -                       err = -ENOMEM;
> > > -       } else
> > > -               err = -ENOENT;                  /* unused swap entry */
> > > -
> > > -       if (!err)
> > > -               WRITE_ONCE(p->swap_map[offset], count | has_cache);
> > > +               has_cache[i] = count[i] & SWAP_HAS_CACHE;
> > > +               count[i] &= ~SWAP_HAS_CACHE;
> > > +               err = 0;
> > > +
> > > +               if (usage == SWAP_HAS_CACHE) {
> > > +
> > > +                       /* set SWAP_HAS_CACHE if there is no cache and entry is used */
> > > +                       if (!has_cache[i] && count[i])
> > > +                               has_cache[i] = SWAP_HAS_CACHE;
> > > +                       else if (has_cache[i])          /* someone else added cache */
> > > +                               err = -EEXIST;
> > > +                       else                            /* no users remaining */
> > > +                               err = -ENOENT;
> > > +               } else if (count[i] || has_cache[i]) {
> > > +
> > > +                       if ((count[i] & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> > > +                               count[i] += usage;
> > > +                       else if ((count[i] & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> > > +                               err = -EINVAL;
> > > +                       else if (swap_count_continued(p, offset + i, count[i]))
> > > +                               count[i] = COUNT_CONTINUED;
> > > +                       else
> > > +                               err = -ENOMEM;
> > > +               } else
> > > +                       err = -ENOENT;                  /* unused swap entry */
> > > +       }
> > >
> > > +       if (!err) {
> > > +               for (i = 0; i < nr; i++)
> > > +                       WRITE_ONCE(p->swap_map[offset + i], count[i] | has_cache[i]);
>
> Here is the place where we really write data. Before that, we only
> touched temp variables.
>
> > > +       }
> > >  unlock_out:
> > >         unlock_cluster_or_swap_info(p, ci);
> > >         return err;
> > >  }
>
> thanks
> Barry