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[54.84.120.176]) by smtp-relay.gmail.com with ESMTPS id ada2fe7eead31-7575b860c6fsm318490137.18.2026.07.30.18.09.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2026 18:09:51 -0700 (PDT) X-Relaying-Domain: arista.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=arista.com; s=Arista-A; t=1785460190; bh=e4YhryDmCXAdMa371dc3J+Gw1lOXK58NsnYTGb1lLpE=; h=From:To:Cc:Subject:Date:From; b=O4Qw18lWDiQL8Ys/wSXOZf1Qbqza8ySrnPXD6DmHtmterPmzKdm+xzGS4SwRkqp37 o633UhREFEa9+JIwCgZAU2rnfbkMSLvuYu0d1cUJF+MVABnsy0dXDoM/wANrWzocDb 4fOwW0ClyifB+krVpQXXhS2pWcwqwvyyFjYO1O8lrbtbClhH2S/ZuZYcRN6108psgO TDiDEzr/tg6oC9Z8eVadSPnRILucbt93TH+gnYM+PQVuFufG4BsHvhlUk0dlsDP9ue 5ZR7xPI886DLDWqnnleJ1ceZn7X/8bA/BUNkRcTxbwbDY5GTAd23asGqxAruJaluU+ 4MtUjda+PL26w== Received: from kevmitch-g-eos-trunk-2026-07-17.sjc.aristanetworks.com (kevmitch-g-eos-trunk-2026-07-17-us117.sjc.aristanetworks.com [10.242.248.27]) by smtp.aristanetworks.com (Postfix) with ESMTP id 836FEC271E; Fri, 31 Jul 2026 01:09:49 +0000 (UTC) X-SMTP-Authentication: Allow-List-permitted X-SMTP-Authentication: Allow-List-permitted From: Kevin Mitchell To: Tejun Heo , Greg Kroah-Hartman Cc: Kevin Mitchell , Chengming Zhou , driver-core@lists.linux.dev, linux-kernel@vger.kernel.org Subject: [PATCH v2] kernfs: fix race to increment for nr_mmapped in kernfs_fop_mmap() Date: Thu, 30 Jul 2026 18:09:12 -0700 Message-ID: <20260731010914.233067-2-kevmitch@arista.com> Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Counts of files to be released (nr_to_release) and mmapped (nr_mmapped) files were added to kernfs_open_node in commit bdb2fd7fc56e ("kernfs: Skip kernfs_drain_open_files() more aggressively") to optimize kernfs_drain_open_files(). A WARN_ON_ONCE sanity check was also added in kernfs_drain_open_files() to ensure that these counters were brought to zero once all files had been drained. Modifications to these counters were protected by kernfs_open_file_mutex everywhere except for in kernfs_fop_mmap(). This caused a race condition where some nr_mmapped increments could get overwritten even while the correct number of kernfs_open_files with mmapped == true were present in the kernfs_open_node's files list. Consequently, the iteration in kernfs_drain_open_files() would underflow nr_mmapped and the WARNING would fire. To fix this, acquire kernfs_open_file_mutex around nr_mmapped updates in kernfs_fop_mmap. The nesting of->mutex -> kernfs_open_file_mutex is safe as kernfs_open_file_mutex is acquired last avoiding the possible cycles highlighted in commit f83f3c515654 ("kernfs: fix locking around kernfs_ops->release() callback"). Fixes: bdb2fd7fc56e ("kernfs: Skip kernfs_drain_open_files() more aggressively") Signed-off-by: Kevin Mitchell --- Changes in v2: - Rework the approach to use kernfs_open_file_mutex_lock instead of atomics based on feedback from Greg KH. - Link to v1: https://lore.kernel.org/all/20251119191758.612694-2-kevmitch@arista.com fs/kernfs/file.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c index 8e0e90c93372..6f63eb6be1cd 100644 --- a/fs/kernfs/file.c +++ b/fs/kernfs/file.c @@ -495,8 +495,12 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma) rc = 0; if (!of->mmapped) { - of->mmapped = true; + struct mutex *mutex = kernfs_open_file_mutex_lock(of->kn); + of_on(of)->nr_mmapped++; + mutex_unlock(mutex) +; + of->mmapped = true; of->vm_ops = vma->vm_ops; } vma->vm_ops = &kernfs_vm_ops; -- 2.51.0