Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race path

[Johannes Weiner]

On Thu, Jan 25, 2024 at 08:51:27AM +0000, Yosry Ahmed wrote:
> In zswap_writeback_entry(), after we get a folio from
> __read_swap_cache_async(), we grab the tree lock again to check that the
> swap entry was not invalidated and recycled. If it was, we delete the
> folio we just added to the swap cache and exit.
> 
> However, __read_swap_cache_async() returns the folio locked when it is
> newly allocated, which is always true for this path, and the folio is
> ref'd. Make sure to unlock and put the folio before returning.
> 
> This was discovered by code inspection, probably because this path
> handles a race condition that should not happen often, and the bug would
> not crash the system, it will only strand the folio indefinitely.
> 
> Fixes: 04fc7816089c ("mm: fix zswap writeback race condition")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

Ouch, good catch.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>