Re: [PATCH] mm: zswap: don't fail a large-folio swapin whose range is not in zswap

[Yosry Ahmed]

On Mon, Sep 7, 2026 at 9:20 AM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>
> thp_swapin_suitable_orders() and shmem_swap_alloc_folio() sample
> zswap_never_enabled() to decide whether a swapin may use a large folio.
> zswap_load() samples the same one-way static key again once the read
> reaches it.  Nothing serialises the two reads, and in between the task
> allocates and pins a high-order folio, which can sleep.
>
> If zswap is enabled for the first time in that window, a large folio that
> was correctly permitted reaches zswap_load(), which rejects every large
> folio with -EINVAL.  swap_read_folio() treats anything other than -ENOENT
> as "zswap handled it" and skips the backing-device read, so the folio
> comes back unlocked and not uptodate: SIGBUS for an anonymous fault, -EIO
> for shmem.  The data is intact on the swap device - it was written there
> before zswap was ever enabled - and the not-uptodate folio stays in the
> swap cache, so every retry of the fault fails the same way.  With
> panic_on_warn the WARN takes the machine down rather than the task.
>
> Scan the range instead of rejecting the folio.  The caller has pinned
> every slot before issuing the read, so zswap cannot start a store or a
> writeback into the range and the scan is stable.  If nothing in the range
> is in zswap it is all on the backing device: return -ENOENT and let
> swap_read_folio() read it.
>
> A range that does have a slot in zswap is still refused, because zswap
> stores large folios as order-0 entries and cannot reconstruct one.  That
> stays reachable - a slot shared with another task can be stored inside
> the same window - and refusing is correct, since the alternative is
> returning the stale device copy.  Report it as -EIO rather than -EINVAL:
> the request is valid, zswap just cannot serve it.  The only caller
> distinguishes -ENOENT from everything else, so that part is a
> documentation fix.
>
> Fixes: 242d12c98174 ("mm: support large folios swap-in for sync io devices")
> Cc: stable@xxxxxxxxxxxxxxx
> Co-developed-by: Alexandre Ghiti <alex@xxxxxxxx>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Acked-by: Yosry Ahmed <yosry@xxxxxxxxxx>


> ---
>  mm/zswap.c | 58 ++++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 43 insertions(+), 15 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 37f34e406c8e3..fd36ac38e9a1e 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1571,6 +1571,32 @@ bool zswap_store(struct folio *folio)
>         return ret;
>  }
>
> +/**
> + * zswap_is_present() - is any slot in [entry, entry + nr) in zswap?
> + * @entry: base swap entry of the range
> + * @nr: number of contiguous slots to check
> + *
> + * Context: The caller must keep the range pinned, otherwise the answer can
> + * change under it.
> + * Return: true if at least one slot in the range is in zswap.
> + */
> +static bool zswap_is_present(swp_entry_t entry, unsigned int nr)
> +{
> +       pgoff_t offset = swp_offset(entry);
> +       struct xarray *tree = swap_zswap_tree(entry);
> +       unsigned long index = offset;
> +
> +       /*
> +        * A pinned range is at most SWAPFILE_CLUSTER slots and is aligned to
> +        * its own size, so one tree covers all of it and a single lookup is
> +        * enough. Scanning only part of the range would report a false
> +        * "absent" and let the caller read a stale copy from the device.
> +        */
> +       BUILD_BUG_ON(SWAPFILE_CLUSTER > ZSWAP_ADDRESS_SPACE_PAGES);
> +
> +       return xa_find(tree, &index, offset + nr - 1, XA_PRESENT);
> +}
> +
>  /**
>   * zswap_load() - load a folio from zswap
>   * @folio: folio to load
> @@ -1578,15 +1604,12 @@ bool zswap_store(struct folio *folio)
>   * Return: 0 on success, with the folio unlocked and marked up-to-date, or one
>   * of the following error codes:
>   *
> - *  -EIO: if the swapped out content was in zswap, but could not be loaded
> - *  into the page due to a decompression failure. The folio is unlocked, but
> - *  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 the swapped out content was in zswap but could not be handed
> + *  back, either because decompression failed or because a slot in a
> + *  large-folio range is still in zswap and zswap cannot reconstruct a large
> + *  folio from per-page entries. The folio is unlocked, but NOT marked
> + *  up-to-date, so that an IO error is emitted (e.g. do_swap_page() will
> + *  SIGBUS).
>   *
>   *  -ENOENT: if the swapped out content was not in zswap. The folio remains
>   *  locked on return.
> @@ -1605,13 +1628,18 @@ 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 can legitimately reach zswap_load() with its whole
> +        * range on the backing device, so scan the range rather than rejecting
> +        * it outright. The caller has pinned every slot, so zswap cannot start
> +        * a store or a writeback into the range while we look.
>          */
> -       if (WARN_ON_ONCE(folio_test_large(folio))) {
> -               folio_unlock(folio);
> -               return -EINVAL;
> +       if (folio_test_large(folio)) {
> +               if (WARN_ON_ONCE(zswap_is_present(swp,
> +                                                 folio_nr_pages(folio)))) {
> +                       folio_unlock(folio);
> +                       return -EIO;
> +               }
> +               return -ENOENT;
>         }
>
>         entry = xa_load(tree, offset);
> --
> 2.53.0-Meta
>