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Message-ID: <CAJD7tkYWvxeCLmDO83tGyde7e4wPDPMnNbQfnB+pyTJAoSHqbg@mail.gmail.com>
Date: Wed, 28 Feb 2024 16:51:45 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Barry Song <21cnbao@...il.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 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];
I am not closely following this series, but a couple of things caught my eyes.
Is this reasonable for stack usage?
> + 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?
>
> - } 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]);
> + }
> unlock_out:
> unlock_cluster_or_swap_info(p, ci);
> return err;
> }
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