+ mm-hugetlb_cgroup-call-page_counter_set_max-outside-vm_bug_on.patch added to mm-hotfixes-unstable branch

+ mm-hugetlb_cgroup-call-page_counter_set_max-outside-vm_bug_on.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/hugetlb_cgroup: call page_counter_set_max() outside VM_BUG_ON()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-hugetlb_cgroup-call-page_counter_set_max-outside-vm_bug_on.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb_cgroup-call-page_counter_set_max-outside-vm_bug_on.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: Narek Jilavyan <njilav@gmail.com>
Subject: mm/hugetlb_cgroup: call page_counter_set_max() outside VM_BUG_ON()
Date: Mon, 17 Aug 2026 10:34:33 +0000

hugetlb_cgroup_css_alloc() rounds the counter limit down to a multiple of
the huge page size and then applies it inside an assertion:

	VM_BUG_ON(page_counter_set_max(fault, limit));
	VM_BUG_ON(page_counter_set_max(rsvd, limit));

With CONFIG_DEBUG_VM=n, VM_BUG_ON(cond) is BUILD_BUG_ON_INVALID(cond),
i.e.  ((void)(sizeof((__force long)(cond)))), whose operand is never
evaluated.  page_counter_set_max() is not a predicate - it performs
xchg(&counter->max, nr_pages) - so on every non-debug kernel the limit is
never applied and the counters keep page_counter_init()'s
PAGE_COUNTER_MAX.

That is user-visible, because hugetlb_cgroup_read_u64_max() recomputes the
same rounded value and uses equality as its "unlimited" sentinel. 
PAGE_COUNTER_MAX is LONG_MAX / PAGE_SIZE = 2251799813685247, which is odd,
so round_down() really does change it and the two sides disagree.  With
CONFIG_DEBUG_VM=n:

	$ cat /sys/fs/cgroup/t/hugetlb.2MB.max
	9223372036854771712

and with this patch:

	$ cat /sys/fs/cgroup/t/hugetlb.2MB.max
	max

A debug option should not change cgroup output.

Call the function, then assert the result, as v6.12 did.  Use
VM_WARN_ON_ONCE() rather than restoring VM_BUG_ON(): the two are identical
under CONFIG_DEBUG_VM=n, and checkpatch asks that new code not use BUG()
variants.

Link: https://lore.kernel.org/20260817103433.191266-1-njilav@gmail.com
Fixes: 0e2759afcaf9 ("page_counter: track failcnt only for legacy cgroups")
Signed-off-by: Narek Jilavyan <njilav@gmail.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb_cgroup.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/mm/hugetlb_cgroup.c~mm-hugetlb_cgroup-call-page_counter_set_max-outside-vm_bug_on
+++ a/mm/hugetlb_cgroup.c
@@ -97,6 +97,7 @@ static void hugetlb_cgroup_init(struct h
 		struct page_counter *fault, *fault_parent = NULL;
 		struct page_counter *rsvd, *rsvd_parent = NULL;
 		unsigned long limit;
+		int ret;
 
 		if (parent_h_cgroup) {
 			fault_parent = hugetlb_cgroup_counter_from_cgroup(
@@ -118,8 +119,10 @@ static void hugetlb_cgroup_init(struct h
 		limit = round_down(PAGE_COUNTER_MAX,
 				   pages_per_huge_page(&hstates[idx]));
 
-		VM_BUG_ON(page_counter_set_max(fault, limit));
-		VM_BUG_ON(page_counter_set_max(rsvd, limit));
+		ret = page_counter_set_max(fault, limit);
+		VM_WARN_ON_ONCE(ret);
+		ret = page_counter_set_max(rsvd, limit);
+		VM_WARN_ON_ONCE(ret);
 	}
 }
 
_

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

mm-hugetlb_cgroup-call-page_counter_set_max-outside-vm_bug_on.patch