+ mm-page_alloc-warn-on-nr_reserved_highatomic-underflow.patch added to mm-unstable branch

+ mm-page_alloc-warn-on-nr_reserved_highatomic-underflow.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/page_alloc: warn on nr_reserved_highatomic underflow
has been added to the -mm mm-unstable branch.  Its filename is
     mm-page_alloc-warn-on-nr_reserved_highatomic-underflow.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-warn-on-nr_reserved_highatomic-underflow.patch

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

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------------------------------------------------------
From: Brendan Jackman <jackmanb@google.com>
Subject: mm/page_alloc: warn on nr_reserved_highatomic underflow
Date: Tue, 25 Feb 2025 18:45:09 +0000

As documented in the comment this underflow should not happen.  The
locking has indeed changed here since the comment was written, see the
migratetype hygiene patches[0].  However, those changes made the locking
_safer_, so the underflow _really_ shouldn't happen now.  So upgrade the
comment to a warning.

[0] https://lore.kernel.org/all/20240320180429.678181-7-hannes@cmpxchg.org/T/#m3da87e6cc3348a4640aa298137bc9f8f61b76c84

Link: https://lkml.kernel.org/r/20250225-warn-underflow-v1-1-3dc542941d3a@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/mm/page_alloc.c~mm-page_alloc-warn-on-nr_reserved_highatomic-underflow
+++ a/mm/page_alloc.c
@@ -3095,6 +3095,7 @@ static bool unreserve_highatomic_pageblo
 			if (!page)
 				continue;
 
+			size = max(pageblock_nr_pages, 1UL << order);
 			/*
 			 * It should never happen but changes to
 			 * locking could inadvertently allow a per-cpu
@@ -3102,8 +3103,8 @@ static bool unreserve_highatomic_pageblo
 			 * while unreserving so be safe and watch for
 			 * underflows.
 			 */
-			size = max(pageblock_nr_pages, 1UL << order);
-			size = min(size, zone->nr_reserved_highatomic);
+			if (WARN_ON_ONCE(size > zone->nr_reserved_highatomic))
+				size = zone->nr_reserved_highatomic;
 			zone->nr_reserved_highatomic -= size;
 
 			/*
_

Patches currently in -mm which might be from jackmanb@google.com are

mm-mmu_gather-update-comment-on-rcu-freeing.patch
selftests-mm-report-errno-when-things-fail-in-gup_longterm.patch
selftests-mm-fix-assumption-that-sudo-is-present.patch
selftests-mm-skip-uffd-stress-if-userfaultfd-not-available.patch
selftests-mm-skip-uffd-wp-mremap-if-userfaultfd-not-available.patch
selftests-mm-uffd-rename-nr_cpus-nr_threads.patch
selftests-mm-print-some-details-when-uffd-stress-gets-bad-params.patch
selftests-mm-dont-fail-uffd-stress-if-too-many-cpus.patch
selftests-mm-skip-map_populate-on-weird-filesystems.patch
selftests-mm-skip-gup_longerm-tests-on-weird-filesystems.patch
mm-page_alloc-warn-on-nr_reserved_highatomic-underflow.patch
scripts-gdb-add-lx_per_cpu_ptr.patch