Re: [PATCH 09/15] mm/memfd: recheck the page cache before allocating a hugetlb folio

[Muchun Song]

> On Oct 29, 2026, at 04:13, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> A concurrent fault or PIN request can populate the page cache
> after memfd_pin_folios() misses it in the page cache but before
> memfd_alloc_folio() takes the fault mutex. The latter still tries to
> allocate another folio and can return -ENOMEM even though the requested
> folio is already cached.
> 
> In particular, hugetlb_reserve_pages() finds the existing reservation-map
> entry and reserves nothing. If no global reservations remain,
> alloc_hugetlb_folio_reserve() fails even when the pool has free pages,
> effectively failing memfd_pin_folios().
> 
> Recheck the page cache before reserving or allocating. If the folio is
> present, drop the lookup reference and return -EEXIST so
> memfd_pin_folios() retries its lookup.
> 
> This is probably not worth backporting to stable.
> 
> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
> Reported-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.