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

From: Yosry Ahmed
Date: Wed Feb 28 2024 - 20:12:27 EST

On Wed, Feb 28, 2024 at 4:59 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Thu, Feb 29, 2024 at 1:52 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > On Wed, Feb 28, 2024 at 4:39 PM Barry Song <21cnbao@xxxxxxxxx> 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: Yu Zhao <yuzhao@xxxxxxxxxx>
> > > 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 | 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