+ shmem-clamp-negative-used-block-samples-in-statfs.patch added to mm-nonmm-unstable branch

+ shmem-clamp-negative-used-block-samples-in-statfs.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: shmem: clamp negative used-block samples in statfs
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     shmem-clamp-negative-used-block-samples-in-statfs.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/shmem-clamp-negative-used-block-samples-in-statfs.patch

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

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------------------------------------------------------
From: Hungyu Lin <dennylin0707@gmail.com>
Subject: shmem: clamp negative used-block samples in statfs
Date: Wed, 28 Oct 2026 05:18:26 +0000

percpu_counter_sum() holds the counter lock while scanning the per-CPU
values, but local updates can still proceed. Concurrent allocation and
freeing on different CPUs can therefore produce a negative sample even
though the number of used tmpfs blocks cannot be negative.

shmem_statfs() subtracts this sample from max_blocks, reporting more free
blocks than the filesystem's total. With a one-page file being written
and truncated on separate CPUs, GNU df reports negative used space, for
example 67108864 total bytes, -28672 used and 67137536 available.

Use percpu_counter_sum_positive() to clamp a negative sample to zero.
This keeps the existing best-effort accounting in statfs and does not
change allocation or limit enforcement.

Link: https://lore.kernel.org/20261028051826.23058-1-dennylin0707@gmail.com
Fixes: 7e496299d4d2 ("tmpfs: make tmpfs scalable with percpu_counter for used blocks")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: <stable@vger.kernel.org>
---

 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/shmem.c~shmem-clamp-negative-used-block-samples-in-statfs
+++ a/mm/shmem.c
@@ -4018,7 +4018,7 @@ static int shmem_statfs(struct dentry *d
 		buf->f_blocks = sbinfo->max_blocks;
 		buf->f_bavail =
 		buf->f_bfree  = sbinfo->max_blocks -
-				percpu_counter_sum(&sbinfo->used_blocks);
+				percpu_counter_sum_positive(&sbinfo->used_blocks);
 	}
 	if (sbinfo->max_inodes) {
 		buf->f_files = sbinfo->max_inodes;
_

Patches currently in -mm which might be from dennylin0707@gmail.com are

mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares.patch
mm-migrate-untag-addresses-when-querying-page-nodes.patch
fat-fix-parent-link-count-when-replacing-a-sibling-directory.patch
delayacct-preserve-delay-extrema-when-aggregating-taskstats.patch
shmem-clamp-negative-used-block-samples-in-statfs.patch