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

[Nhat Pham]

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: Nhat Pham <nphamcs@xxxxxxxxx>