[PATCH v2] kernfs: fix race to increment for nr_mmapped in kernfs_fop_mmap()

[Kevin Mitchell <kevmitch@arista.com>]

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 <kevmitch@arista.com>
---
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