Re: [PATCH] mm/hugetlb: fix subpool minimum reservation rollback

[Muchun Song]

> On Sep 7, 2026, at 21:20, Jinmeng Zhou <jinmengzhou22@xxxxxxxxx> wrote:
> 
> When a reservation request is partially covered by a subpool minimum and
> the remaining global reservation fails, the error path first calls
> hugepage_subpool_put_pages() for the subpool-backed portion. It removes the
> failed global portion from used_hpages only afterwards.
> 
> hugepage_subpool_put_pages() uses used_hpages to decide whether rsv_hpages
> should be restored. Since used_hpages still includes the global portion, it
> can remain at or above min_hpages and prevent that restoration. It then
> reports the subpool reservation as releasable, causing
> hugetlb_acct_memory() to incorrectly decrement h->resv_huge_pages.
> 
> This was reproduced with four 2 MB huge pages and a hugetlbfs mount with
> size=10M,min_size=8M. After a successful three-page reservation, a two-page
> reservation which needed one subpool page and one global page failed with
> -ENOMEM. HugePages_Rsvd incorrectly dropped from four to three even though
> the subpool minimum was still four pages.
> 
> Roll back the failed global portion from used_hpages first, so that
> hugepage_subpool_put_pages() evaluates the minimum reservation against the
> current usage and returns the correct global adjustment.
> 
> Fixes: a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jinmeng Zhou <zhoujinmeng@xxxxxxxxxxxxx>

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

Thanks.