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Message-ID: <CAGsJ_4wSDZM4s6v8KmjwjbpWcoeF95R3-AkhUt2SthZ49MNNcw@mail.gmail.com>
Date: Thu, 29 Feb 2024 13:58:56 +1300
From: Barry Song <21cnbao@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, linux-mm@...ck.org,
ryan.roberts@....com, chrisl@...nel.org, linux-kernel@...r.kernel.org,
mhocko@...e.com, shy828301@...il.com, steven.price@....com, surenb@...gle.com,
wangkefeng.wang@...wei.com, willy@...radead.org, xiang@...nel.org,
ying.huang@...el.com, yuzhao@...gle.com, kasong@...cent.com,
nphamcs@...il.com, chengming.zhou@...ux.dev, hannes@...xchg.org,
linux-arm-kernel@...ts.infradead.org, Barry Song <v-songbaohua@...o.com>,
Hugh Dickins <hughd@...gle.com>, Minchan Kim <minchan@...nel.org>, SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH RFC v2 4/5] mm: swap: introduce swapcache_prepare_nr and
swapcache_clear_nr for large folios swap-in
On Thu, Feb 29, 2024 at 1:52 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Wed, Feb 28, 2024 at 4:39 PM Barry Song <21cnbao@...il.com> wrote:
> >
> > From: Barry Song <v-songbaohua@...o.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@...cent.com>
> > Cc: "Huang, Ying" <ying.huang@...el.com>
> > Cc: Yu Zhao <yuzhao@...gle.com>
> > Cc: David Hildenbrand <david@...hat.com>
> > Cc: Chris Li <chrisl@...nel.org>
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: Yosry Ahmed <yosryahmed@...gle.com>
> > Cc: Yu Zhao <yuzhao@...gle.com>
> > Cc: SeongJae Park <sj@...nel.org>
> > Signed-off-by: Barry Song <v-songbaohua@...o.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.
#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.
>
> >
> > - } 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
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