Re: [PATCH v8 1/3] mm: Fix swap_read_folio_zeromap() for large folios with partial zeromap

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Sep 5, 2024 at 5:11 PM Barry Song <21cnbao@gmail.com> wrote:
>
> From: Barry Song <v-songbaohua@oppo.com>
>
> There could be a corner case where the first entry is non-zeromap,
> but a subsequent entry is zeromap. In this case, we should not
> let swap_read_folio_zeromap() return false since we will still
> read corrupted data.
>
> Additionally, the iteration of test_bit() is unnecessary and
> can be replaced with bitmap operations, which are more efficient.
>
> We can adopt the style of swap_pte_batch() and folio_pte_batch() to
> introduce swap_zeromap_batch() which seems to provide the greatest
> flexibility for the caller. This approach allows the caller to either
> check if the zeromap status of all entries is consistent or determine
> the number of contiguous entries with the same status.
>
> Since swap_read_folio() can't handle reading a large folio that's
> partially zeromap and partially non-zeromap, we've moved the code
> to mm/swap.h so that others, like those working on swap-in, can
> access it.
>
> Fixes: 0ca0c24e3211 ("mm: store zero pages to be swapped out in a bitmap")
> Cc: Usama Arif <usamaarif642@gmail.com>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>  mm/page_io.c | 32 +++++++-------------------------
>  mm/swap.h    | 33 +++++++++++++++++++++++++++++++++
>  2 files changed, 40 insertions(+), 25 deletions(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 4bc77d1c6bfa..2dfe2273a1f1 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -226,26 +226,6 @@ static void swap_zeromap_folio_clear(struct folio *folio)
>         }
>  }
>
> -/*
> - * Return the index of the first subpage which is not zero-filled
> - * according to swap_info_struct->zeromap.
> - * If all pages are zero-filled according to zeromap, it will return
> - * folio_nr_pages(folio).
> - */
> -static unsigned int swap_zeromap_folio_test(struct folio *folio)
> -{
> -       struct swap_info_struct *sis = swp_swap_info(folio->swap);
> -       swp_entry_t entry;
> -       unsigned int i;
> -
> -       for (i = 0; i < folio_nr_pages(folio); i++) {
> -               entry = page_swap_entry(folio_page(folio, i));
> -               if (!test_bit(swp_offset(entry), sis->zeromap))
> -                       return i;
> -       }
> -       return i;
> -}
> -
>  /*
>   * We may have stale swap cache pages in memory: notice
>   * them here and get rid of the unnecessary final write.
> @@ -524,19 +504,21 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>
>  static bool swap_read_folio_zeromap(struct folio *folio)
>  {
> -       unsigned int idx = swap_zeromap_folio_test(folio);
> -
> -       if (idx == 0)
> -               return false;
> +       int nr_pages = folio_nr_pages(folio);
> +       bool is_zeromap;
> +       int nr_zeromap = swap_zeromap_batch(folio->swap, nr_pages, &is_zeromap);

swap_zeromap_batch() reads to me like the number of entries that are
in the zeromap (i.e. bits are set), not the number of contiguous equal
bits. I can't think of a better name though :/

The local variable is not adding much value here either. It's
reinforcing the misunderstanding I point out above, if anything. You
can just drop that.

>
>         /*
>          * Swapping in a large folio that is partially in the zeromap is not
>          * currently handled. 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(idx < folio_nr_pages(folio)))
> +       if (WARN_ON_ONCE(nr_zeromap != nr_pages))
>                 return true;
>
> +       if (!is_zeromap)
> +               return false;
> +
>         folio_zero_range(folio, 0, folio_size(folio));
>         folio_mark_uptodate(folio);
>         return true;
> diff --git a/mm/swap.h b/mm/swap.h
> index f8711ff82f84..1cc56a02fb5f 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -80,6 +80,32 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
>  {
>         return swp_swap_info(folio->swap)->flags;
>  }
> +
> +/*
> + * Return the count of contiguous swap entries that share the same
> + * zeromap status as the starting entry. If is_zeromap is not NULL,
> + * it will return the zeromap status of the starting entry.
> + */
> +static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr,
> +               bool *is_zeromap)
> +{
> +       struct swap_info_struct *sis = swp_swap_info(entry);
> +       unsigned long start = swp_offset(entry);
> +       unsigned long end = start + max_nr;
> +       bool start_entry_zeromap;
> +
> +       start_entry_zeromap = test_bit(start, sis->zeromap);

first_bit is probably a better name.

> +       if (is_zeromap)
> +               *is_zeromap = start_entry_zeromap;
> +
> +       if (max_nr <= 1)
> +               return max_nr;
> +       if (start_entry_zeromap)
> +               return find_next_zero_bit(sis->zeromap, end, start) - start;
> +       else
> +               return find_next_bit(sis->zeromap, end, start) - start;

The usage of these functions look correct to me, although
FIND_NEXT_BIT is not really easy for me to parse :)