Re: [PATCH 12/15] mm/hugetlb: keep surplus folios that are reserved

[Muchun Song]

> On Oct 29, 2026, at 04:13, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> From: Chris Mason <mason@xxxxxxxxxx>
> 
> free_huge_folio() may restore a reservation, but still returns the folio
> to the page allocator if its node has surplus huge pages.
> 
> For example, start with no hugetlb folios and allow one surplus 2 MiB
> folio:
> 
>  1. Mount hugetlbfs with pagesize=2M,min_size=2M. This allocates and
>     reserves one surplus folio: free=1, reserved=1.
>  2. Create a 2 MiB file and write to a shared mapping of it. The fault
>     consumes the folio and its reservation: free=0, reserved=0.
>  3. Unmap the file and truncate it to zero. The mount's reservation is
>     restored, but the surplus folio is freed: free=0, reserved=1.
> 
> Besides losing a hugetlb folio that is required for a reservation, this
> can also make available_huge_pages() underflow and cause other issues.
> 
> Keep the folio in the pool if it is needed to cover reservations,
> including any reservation we just restored.
> 
> Fix generated by an LLM and sent by Chris Mason in private.
> 
> Fixes: 07443a85ad90 ("mm, hugetlb: return a reserved page to a reserved pool if failed")
> Cc: stable@xxxxxxxxxx
> Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
> [ david: simplify comment and description ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

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

Thanks.