Re: [PATCH] mm: swap: mTHP frees entries as a whole

[zhiguojiang]

在 2024/8/6 6:09, Barry Song 写道:
> On Tue, Aug 6, 2024 at 4:08 AM Zhiguo Jiang <justinjiang@xxxxxxxx> wrote:
> > Support mTHP's attempt to free swap entries as a whole, which can avoid
> > frequent swap_info locking for every individual entry in
> > swapcache_free_entries(). When the swap_map count values corresponding
> > to all contiguous entries are all zero excluding SWAP_HAS_CACHE, the
> > entries will be freed directly by skippping percpu swp_slots caches.
> No, this isn't quite good. Please review the work done by Chris and Kairui[1];
> they have handled it better. On a different note, I have a patch that can
> handle zap_pte_range() for swap entries in batches[2][3].
I'm glad to see your optimized submission about batch freeing swap 
entries for
zap_pte_range(), sorry, I didn't see it before. My this patch can be 
ignored.

Thanks
Zhiguo

> [1] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-5-cb9c148b9297@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20240803091118.84274-1-21cnbao@xxxxxxxxx/
> [3] https://lore.kernel.org/linux-mm/CAGsJ_4wPnQqKOHx6iQcwO8bQzoBXKr2qY2AgSxMwTQCj3-8YWw@xxxxxxxxxxxxxx/
>
> > Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
> > ---
> >   mm/swapfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 61 insertions(+)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index ea023fc25d08..829fb4cfb6ec
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1493,6 +1493,58 @@ static void swap_entry_range_free(struct swap_info_struct *p, swp_entry_t entry,
> >          swap_range_free(p, offset, nr_pages);
> >   }
> >
> > +/*
> > + * Free the contiguous swap entries as a whole, caller have to
> > + * ensure all entries belong to the same folio.
> > + */
> > +static void swap_entry_range_check_and_free(struct swap_info_struct *p,
> > +                                 swp_entry_t entry, int nr, bool *any_only_cache)
> > +{
> > +       const unsigned long start_offset = swp_offset(entry);
> > +       const unsigned long end_offset = start_offset + nr;
> > +       unsigned long offset;
> > +       DECLARE_BITMAP(to_free, SWAPFILE_CLUSTER) = { 0 };
> > +       struct swap_cluster_info *ci;
> > +       int i = 0, nr_setbits = 0;
> > +       unsigned char count;
> > +
> > +       /*
> > +        * Free and check swap_map count values corresponding to all contiguous
> > +        * entries in the whole folio range.
> > +        */
> > +       WARN_ON_ONCE(nr > SWAPFILE_CLUSTER);
> > +       ci = lock_cluster_or_swap_info(p, start_offset);
> > +       for (offset = start_offset; offset < end_offset; offset++, i++) {
> > +               if (data_race(p->swap_map[offset])) {
> > +                       count = __swap_entry_free_locked(p, offset, 1);
> > +                       if (!count) {
> > +                               bitmap_set(to_free, i, 1);
> > +                               nr_setbits++;
> > +                       } else if (count == SWAP_HAS_CACHE) {
> > +                               *any_only_cache = true;
> > +                       }
> > +               } else {
> > +                       WARN_ON_ONCE(1);
> > +               }
> > +       }
> > +       unlock_cluster_or_swap_info(p, ci);
> > +
> > +       /*
> > +        * If the swap_map count values corresponding to all contiguous entries are
> > +        * all zero excluding SWAP_HAS_CACHE, the entries will be freed directly by
> > +        * skippping percpu swp_slots caches, which can avoid frequent swap_info
> > +        * locking for every individual entry.
> > +        */
> > +       if (nr > 1 && nr_setbits == nr) {
> > +               spin_lock(&p->lock);
> > +               swap_entry_range_free(p, entry, nr);
> > +               spin_unlock(&p->lock);
> > +       } else {
> > +               for_each_set_bit(i, to_free, SWAPFILE_CLUSTER)
> > +                       free_swap_slot(swp_entry(p->type, start_offset + i));
> > +       }
> > +}
> > +
> >   static void cluster_swap_free_nr(struct swap_info_struct *sis,
> >                  unsigned long offset, int nr_pages,
> >                  unsigned char usage)
> > @@ -1808,6 +1860,14 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> >          if (WARN_ON(end_offset > si->max))
> >                  goto out;
> >
> > +       /*
> > +        * Try to free all contiguous entries about mTHP as a whole.
> > +        */
> > +       if (IS_ENABLED(CONFIG_THP_SWAP) && nr > 1) {
> > +               swap_entry_range_check_and_free(si, entry, nr, &any_only_cache);
> > +               goto free_cache;
> > +       }
> > +
> >          /*
> >           * First free all entries in the range.
> >           */
> > @@ -1821,6 +1881,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> >                  }
> >          }
> >
> > +free_cache:
> >          /*
> >           * Short-circuit the below loop if none of the entries had their
> >           * reference drop to zero.
> > --
> > 2.39.0