+ mm-hugetlb-restore-failed-global-reservations-to-subpool.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/hugetlb: restore failed global reservations to subpool
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-hugetlb-restore-failed-global-reservations-to-subpool.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-restore-failed-global-reservations-to-subpool.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Joshua Hahn <joshua.hahnjy@gmail.com>
Subject: mm/hugetlb: restore failed global reservations to subpool
Date: Fri, 16 Jan 2026 15:40:36 -0500

Commit a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
fixed an underflow error for hstate->resv_huge_pages caused by incorrectly
attributing globally requested pages to the subpool's reservation.

Unfortunately, this fix also introduced the opposite problem, which would
leave spool->used_hpages elevated if the globally requested pages could
not be acquired.  This is because while a subpool's reserve pages only
accounts for what is requested and allocated from the subpool, its "used"
counter keeps track of what is consumed in total, both from the subpool
and globally.  Thus, we need to adjust spool->used_hpages in the other
direction, and make sure that globally requested pages are uncharged from
the subpool's used counter.

Each failed allocation attempt increments the used_hpages counter by how
many pages were requested from the global pool.  Ultimately, this renders
the subpool unusable, as used_hpages approaches the max limit.

The issue can be reproduced as follows:
1. Allocate 4 hugetlb pages
2. Create a hugetlb mount with max=4, min=2
3. Consume 2 pages globally
4. Request 3 pages from the subpool (2 from subpool + 1 from global)
	4.1 hugepage_subpool_get_pages(spool, 3) succeeds.
		used_hpages += 3
	4.2 hugetlb_acct_memory(h, 1) fails: no global pages left
		used_hpages -= 2
5. Subpool now has used_hpages = 1, despite not being able to
   successfully allocate any hugepages. It believes it can now only
   allocate 3 more hugepages, not 4.

Repeating this process will ultimately render the subpool unable to
allocate any hugepages, since it believes that it is using the maximum
number of hugepages that the subpool has been allotted.

The underflow issue that the original commit fixes still remains fixed
as well.

Link: https://lkml.kernel.org/r/20260116204037.2270096-1-joshua.hahnjy@gmail.com
Fixes: a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Ma Wupeng <mawupeng1@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Waiman Long <longman@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/mm/hugetlb.c~mm-hugetlb-restore-failed-global-reservations-to-subpool
+++ a/mm/hugetlb.c
@@ -6717,6 +6717,15 @@ out_put_pages:
 		 */
 		hugetlb_acct_memory(h, -gbl_resv);
 	}
+	/* Restore used_hpages for pages that failed global reservation */
+	if (gbl_reserve && spool) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&spool->lock, flags);
+		if (spool->max_hpages != -1)
+			spool->used_hpages -= gbl_reserve;
+		unlock_or_release_subpool(spool, flags);
+	}
 out_uncharge_cgroup:
 	hugetlb_cgroup_uncharge_cgroup_rsvd(hstate_index(h),
 					    chg * pages_per_huge_page(h), h_cg);
_

Patches currently in -mm which might be from joshua.hahnjy@gmail.com are

mm-hugetlb-restore-failed-global-reservations-to-subpool.patch
mm-hugetlb-remove-unnecessary-if-condition.patch
mm-hugetlb-enforce-brace-style.patch