Re: [PATCH v3 20/47] mm/gup: fix clearing folios in memfd_pin_folios()

[Lance Yang]

On Sat, Oct 31, 2026 at 10:39:49AM +0100, David Hildenbrand (Arm) wrote:
>From: Chris Mason <mason@xxxxxxxxxx>
>
>memfd_pin_folios() pins folios obtained directly from the page cache
>without checking whether they are uptodate. However, shmem fallocate()
>can leave newly allocated folios uncleared and not uptodate. Callers
>such as udmabuf can then expose uninitialized memory to userspace.

Right, just make sure the folios are initialized before handing them to
callers. They shouldn't have to care whether shmem deferred the clearing ;)

>Clear non-uptodate folios and mark them uptodate under the folio lock
>before returning them to the caller.
>
>Fix generated by an LLM and sent by Chris Mason in private.
>
>Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
>Cc: stable@xxxxxxxxxxxxxxx
>Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
>[ david: simplify description and comment ]
>Reviewed-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>
>Reviewed-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
>Tested-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
>Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>---

LGTM!

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>