Re: [RFC PATCH v3 4/5] mm: swap: introduce swapcache_prepare_nr and swapcache_clear_nr for large folios swap-in

[Barry Song]

On Wed, Mar 13, 2024 at 4:35 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 04/03/2024 08:13, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Chris Li <chrisl@xxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> > Cc: SeongJae Park <sj@xxxxxxxxxx>
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> >  include/linux/swap.h |   1 +
> >  mm/swap.h            |   1 +
> >  mm/swapfile.c        | 118 ++++++++++++++++++++++++++-----------------
> >  3 files changed, 74 insertions(+), 46 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index d6ab27929458..22105f0fe2d4 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 entry, 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 244106998a69..bae1b8165b11 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3309,7 +3309,7 @@ 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
> > @@ -3319,66 +3319,76 @@ 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)
>
> perhaps its better to pass order instead of nr to all these functions to make it
> clearer that entry should be naturally aligned and be a power-of-2 number of
> pages, no bigger than SWAPFILE_CLUSTER?
>
> >  {
> >       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];
>
> I'm not sure this 1K stack buffer is a good idea?
>
> Could you split it slightly differently so that loop 1 just does error checking
> and bails out if an error is found, and loop 2 does the new value calculation
> and writeback? Then you don't need these arrays.

right. we can totally remove thoe arrays by re-reading swap_map.

>
> > +     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;
> > -     }
> > -
> > -     has_cache = count & SWAP_HAS_CACHE;
> > -     count &= ~SWAP_HAS_CACHE;
> > -     err = 0;
> > -
> > -     if (usage == SWAP_HAS_CACHE) {
> > +     for (i = 0; i < nr; i++) {
> > +             count[i] = p->swap_map[offset + i];
> >
> > -             /* 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;
> > +             }
> >
> > -     } 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 */
> > +             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)
> > -             WRITE_ONCE(p->swap_map[offset], count | has_cache);
> > +             if (err)
> > +                     break;
> > +     }
> >
> > +     if (!err) {
> > +             for (i = 0; i < nr; i++)
> > +                     WRITE_ONCE(p->swap_map[offset + i], count[i] | has_cache[i]);
> > +     }
> >  unlock_out:
> >       unlock_cluster_or_swap_info(p, ci);
> >       return err;
> >  }
> >
> > +static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> > +{
> > +     return __swap_duplicate_nr(entry, 1, usage);
> > +}
> > +
> >  /*
> >   * Help swapoff by noting that swap entry belongs to shmem/tmpfs
> >   * (in which case its reference count is never incremented).
> > @@ -3417,17 +3427,33 @@ int swapcache_prepare(swp_entry_t entry)
> >       return __swap_duplicate(entry, SWAP_HAS_CACHE);
> >  }
> >
> > -void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +int swapcache_prepare_nr(swp_entry_t entry, int nr)
> > +{
> > +     return __swap_duplicate_nr(entry, nr, SWAP_HAS_CACHE);
> > +}
> > +
> > +void swapcache_clear_nr(struct swap_info_struct *si, swp_entry_t entry, int nr)
> >  {
> >       struct swap_cluster_info *ci;
> >       unsigned long offset = swp_offset(entry);
> > -     unsigned char usage;
> > +     unsigned char usage[SWAPFILE_CLUSTER];
> > +     int i;
> >
> >       ci = lock_cluster_or_swap_info(si, offset);
> > -     usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE);
> > +     for (i = 0; i < nr; i++)
> > +             usage[i] = __swap_entry_free_locked(si, offset + i, SWAP_HAS_CACHE);
> >       unlock_cluster_or_swap_info(si, ci);
> > -     if (!usage)
> > -             free_swap_slot(entry);
> > +     for (i = 0; i < nr; i++) {
> > +             if (!usage[i]) {
> > +                     free_swap_slot(entry);
> > +                     entry.val++;
> > +             }
> > +     }
> > +}
>
> This is pretty similar to swap_nr_free() which you added in patch 2. Except
> swap_nr_free() passes 1 as last param to __swap_entry_free_locked() and this
> passes SWAP_HAS_CACHE. Perhaps their should be a common helper? I think
> swap_nr_free()'s usage bitmap is preferable to this version's char array too.

right.

>
> Thanks,
> Ryan
>
> > +
> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > +     swapcache_clear_nr(si, entry, 1);
> >  }
> >
> >  struct swap_info_struct *swp_swap_info(swp_entry_t entry)

Thanks
Barry