Re: [PATCH v3 3/3] mm: zswap: handle incorrect attempts to load large folios

[Barry Song <21cnbao@gmail.com>]

On Tue, Jun 11, 2024 at 2:45 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> Zswap does not support storing or loading large folios. Until proper
> support is added, attempts to load large folios from zswap are a bug.
>
> For example, if a swapin fault observes that contiguous PTEs are
> pointing to contiguous swap entries and tries to swap them in as a large
> folio, swap_read_folio() will pass in a large folio to zswap_load(), but
> zswap_load() will only effectively load the first page in the folio. If
> the first page is not in zswap, the folio will be read from disk, even
> though other pages may be in zswap.
>
> In both cases, this will lead to silent data corruption. Proper support
> needs to be added before large folio swapins and zswap can work
> together.
>
> Looking at callers of swap_read_folio(), it seems like they are either
> allocated from __read_swap_cache_async() or do_swap_page() in the
> SWP_SYNCHRONOUS_IO path. Both of which allocate order-0 folios, so
> everything is fine for now.
>
> However, there is ongoing work to add to support large folio swapins
> [1]. To make sure new development does not break zswap (or get broken by
> zswap), add minimal handling of incorrect loads of large folios to
> zswap.
>
> First, move the call folio_mark_uptodate() inside zswap_load().
>
> If a large folio load is attempted, and zswap was ever enabled on the
> system, return 'true' without calling folio_mark_uptodate(). This will
> prevent the folio from being read from disk, and will emit an IO error
> because the folio is not uptodate (e.g. do_swap_fault() will return
> VM_FAULT_SIGBUS). It may not be reliable recovery in all cases, but it
> is better than nothing.
>
> This was tested by hacking the allocation in __read_swap_cache_async()
> to use order 2 and __GFP_COMP.
>
> In the future, to handle this correctly, the swapin code should:
> (a) Fallback to order-0 swapins if zswap was ever used on the machine,
> because compressed pages remain in zswap after it is disabled.
> (b) Add proper support to swapin large folios from zswap (fully or
> partially).
>
> Probably start with (a) then followup with (b).
>
> [1]https://lore.kernel.org/linux-mm/20240304081348.197341-6-21cnbao@gmail.com/
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

Acked-by: Barry Song <baohua@kernel.org>

> ---
>  mm/page_io.c |  1 -
>  mm/zswap.c   | 12 ++++++++++++
>  2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index f1a9cfab6e748..8f441dd8e109f 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -517,7 +517,6 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
>         delayacct_swapin_start();
>
>         if (zswap_load(folio)) {
> -               folio_mark_uptodate(folio);
>                 folio_unlock(folio);
>         } else if (data_race(sis->flags & SWP_FS_OPS)) {
>                 swap_read_folio_fs(folio, plug);
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 7fcd751e847d6..505f4b9812891 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1566,6 +1566,17 @@ bool zswap_load(struct folio *folio)
>         if (zswap_never_enabled())
>                 return false;
>
> +       /*
> +        * Large folios should not be swapped in while zswap is being used, as
> +        * they are not properly handled. Zswap does not properly load large
> +        * folios, and a large folio may only be partially in zswap.
> +        *
> +        * Return true without marking the folio uptodate so that an IO error is
> +        * emitted (e.g. do_swap_page() will sigbus).
> +        */
> +       if (WARN_ON_ONCE(folio_test_large(folio)))
> +               return true;
> +
>         /*
>          * When reading into the swapcache, invalidate our entry. The
>          * swapcache can be the authoritative owner of the page and
> @@ -1600,6 +1611,7 @@ bool zswap_load(struct folio *folio)
>                 folio_mark_dirty(folio);
>         }
>
> +       folio_mark_uptodate(folio);
>         return true;
>  }
>
> --
> 2.45.2.505.gda0bf45e8d-goog
>