linux-kernel - Re: [PATCH] mm: zswap: fix data loss on SWP_SYNCHRONOUS

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Message-ID: <CAJD7tkaWQAV=X1pzYG=VkWe7Ue9ZFbjt9uQ5m1NJujtLspWJTA@mail.gmail.com>
Date: Sun, 24 Mar 2024 14:22:46 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Zhongkun He <hezhongkun.hzk@...edance.com>, 
	Chengming Zhou <zhouchengming@...edance.com>, Barry Song <21cnbao@...il.com>, 
	Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: zswap: fix data loss on SWP_SYNCHRONOUS_IO devices

On Sun, Mar 24, 2024 at 2:04 PM Johannes Weiner <hannes@...xchg.org> wrote:
>
> Zhongkun He reports data corruption when combining zswap with zram.
>
> The issue is the exclusive loads we're doing in zswap. They assume
> that all reads are going into the swapcache, which can assume
> authoritative ownership of the data and so the zswap copy can go.
>
> However, zram files are marked SWP_SYNCHRONOUS_IO, and faults will try
> to bypass the swapcache. This results in an optimistic read of the
> swap data into a page that will be dismissed if the fault fails due to
> races. In this case, zswap mustn't drop its authoritative copy.
>
> Link: https://lore.kernel.org/all/CACSyD1N+dUvsu8=zV9P691B9bVq33erwOXNTmEaUbi9DrDeJzw@mail.gmail.com/
> Reported-by: Zhongkun He <hezhongkun.hzk@...edance.com>
> Fixes: b9c91c43412f ("mm: zswap: support exclusive loads")
> Cc: stable@...r.kernel.org      [6.5+]
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Tested-by: Zhongkun He <hezhongkun.hzk@...edance.com>

Do we also want to mention somewhere (commit log or comment) that
keeping the entry in the tree is fine because we are still protected
from concurrent loads/invalidations/writeback by swapcache_prepare()
setting SWAP_HAS_CACHE or so?

Anyway, this LGTM.

Acked-by: Yosry Ahmed <yosryahmed@...gle.com>

> ---
>  mm/zswap.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 535c907345e0..41a1170f7cfe 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1622,6 +1622,7 @@ bool zswap_load(struct folio *folio)
>         swp_entry_t swp = folio->swap;
>         pgoff_t offset = swp_offset(swp);
>         struct page *page = &folio->page;
> +       bool swapcache = folio_test_swapcache(folio);
>         struct zswap_tree *tree = swap_zswap_tree(swp);
>         struct zswap_entry *entry;
>         u8 *dst;
> @@ -1634,7 +1635,20 @@ bool zswap_load(struct folio *folio)
>                 spin_unlock(&tree->lock);
>                 return false;
>         }
> -       zswap_rb_erase(&tree->rbroot, entry);
> +       /*
> +        * When reading into the swapcache, invalidate our entry. The
> +        * swapcache can be the authoritative owner of the page and
> +        * its mappings, and the pressure that results from having two
> +        * in-memory copies outweighs any benefits of caching the
> +        * compression work.
> +        *
> +        * (Most swapins go through the swapcache. The notable
> +        * exception is the singleton fault on SWP_SYNCHRONOUS_IO
> +        * files, which reads into a private page and may free it if
> +        * the fault fails. We remain the primary owner of the entry.)
> +        */
> +       if (swapcache)
> +               zswap_rb_erase(&tree->rbroot, entry);
>         spin_unlock(&tree->lock);
>
>         if (entry->length)
> @@ -1649,9 +1663,10 @@ bool zswap_load(struct folio *folio)
>         if (entry->objcg)
>                 count_objcg_event(entry->objcg, ZSWPIN);
>
> -       zswap_entry_free(entry);
> -
> -       folio_mark_dirty(folio);
> +       if (swapcache) {
> +               zswap_entry_free(entry);
> +               folio_mark_dirty(folio);
> +       }
>
>         return true;
>  }
> --
> 2.44.0
>