Re: [PATCH v4 04/11] mm: zswap: add range lookup for large-folio swapin

[Yosry Ahmed]

>  /**
>   * zswap_load() - load a folio from zswap
>   * @folio: folio to load
> @@ -1571,10 +1588,9 @@ bool zswap_store(struct folio *folio)
>   *  NOT marked up-to-date, so that an IO error is emitted (e.g. do_swap_page()
>   *  will SIGBUS).
>   *
> - *  -EINVAL: if the swapped out content was in zswap, but the page belongs
> - *  to a large folio, which is not supported by zswap. The folio is unlocked,
> - *  but NOT marked up-to-date, so that an IO error is emitted (e.g.
> - *  do_swap_page() will SIGBUS).
> + *  -EIO: if a slot in a large-folio range is unexpectedly still in zswap.
> + *  The folio is unlocked, but NOT marked up-to-date, so that an IO
> + *  error is emitted (e.g. do_swap_page() will SIGBUS).

Just curious, why the change from -EINVAL to -EIO here? Does it matter
in practice or is it just a matter of personal taste? :)

>   *
>   *  -ENOENT: if the swapped out content was not in zswap. The folio remains
>   *  locked on return.
> @@ -1593,13 +1609,19 @@ int zswap_load(struct folio *folio)
>                 return -ENOENT;
>
>         /*
> -        * 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.
> +        * A large folio reaches zswap_load() only when its whole range is
> +        * expected to be on disk: PMD swap-entry consumers split before
> +        * calling into PMD-order swapin whenever any slot is still in zswap.
> +        * Confirm the range is entirely absent from zswap and return -ENOENT
> +        * so the caller reads it from disk; if a slot is unexpectedly still in
> +        * zswap, fail the read rather than return partially-initialized data.
>          */
> -       if (WARN_ON_ONCE(folio_test_large(folio))) {
> -               folio_unlock(folio);
> -               return -EINVAL;
> +       if (folio_test_large(folio)) {
> +               if (zswap_is_present(swp, folio_nr_pages(folio))) {
> +                       folio_unlock(folio);
> +                       return -EIO;
> +               }
> +               return -ENOENT;
>         }
>
>         entry = xa_load(tree, offset);
> --
> 2.53.0-Meta
>