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

[Andrew Morton]

On Mon,  7 Sep 2026 21:20:55 +0800 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.

Thanks for making a test case.  Perhaps this one could be enshrined in
a selftests/

> 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.

OK, I'll queue this for testing and review.

AI review might have found a couple of issues in the nearby code:

	https://sashiko.dev/#/patchset/20260907132055.26696-1-zhoujinmeng@xxxxxxxxxxxxx