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

["Gregory Price (Meta)" <gourry@gourry.net>]

On 2026-10-28 21:13 +0100, David Hildenbrand (Arm) wrote:
> From: Chris Mason <mason@kernel.org>
> 
> 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@kernel.org
> Signed-off-by: Chris Mason <mason@kernel.org>
> [ david: simplify comment and description ]
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Was able to reproduce the issue and demonstrate the patch resolves it.

LGTM

Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>

-- 
~Gregory