Re: [PATCH] mm: zswap: fix data loss on SWP_SYNCHRONOUS_IO devices

Chris Li
2024-03-25T21:27:18Z

On Sun, Mar 24, 2024 at 2:04 PM Johannes Weiner <hannes@cmpxchg.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@bytedance.com>
> Fixes: b9c91c43412f ("mm: zswap: support exclusive loads")
> Cc: stable@vger.kernel.org [6.5+]
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Tested-by: Zhongkun He <hezhongkun.hzk@bytedance.com>

The change looks good to me. Thanks for the cleaner solution.
It has conflict with the zswap rb-tree to xarray patch though.
I will resolve the conflict and resubmit the zswap xarray patch.

Acked-by: Chris Li <chrisl@kernel.org>

Chris
> ---
> 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
>