linux-kernel - [PATCH 1/1] loop: sync filesystem cache before getting file size in get

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Message-ID: <20250807232522.192898-1-rajeevm@hpe.com>
Date: Thu,  7 Aug 2025 23:25:22 +0000
From: Rajeev Mishra <rajeevm@....com>
To: axboe@...nel.dk
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rajeev Mishra <rajeevm@....com>
Subject: [PATCH 1/1] loop: sync filesystem cache before getting file size in get_size()

The get_size() function now uses vfs_getattr_nosec() with AT_STATX_SYNC_AS_STAT
to ensure filesystem cache is synchronized before retrieving file size. This
provides more accurate size information, especially when:

- The backing file size has been changed by another process
- The file is on a network filesystem (NFS, CIFS, etc.)
- The file is being modified concurrently
- The most accurate size is needed for loop device setup

The implementation gracefully falls back to i_size_read() if vfs_getattr_nosec()
fails, maintaining backward compatibility.

Signed-off-by: Rajeev Mishra <rajeevm@....com>
---
 drivers/block/loop.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 1b6ee91f8eb9..15d5edbc69ce 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -137,12 +137,39 @@ static void loop_global_unlock(struct loop_device *lo, bool global)
 static int max_part;
 static int part_shift;
 
+/**
+ * get_size - calculate the effective size of a loop device
+ * @offset: offset into the backing file
+ * @sizelimit: user-specified size limit
+ * @file: the backing file
+ *
+ * Calculate the effective size of the loop device
+ *
+ * Returns: size in 512-byte sectors, or 0 if invalid
+ */
 static loff_t get_size(loff_t offset, loff_t sizelimit, struct file *file)
 {
+	struct kstat stat;
 	loff_t loopsize;
+	int ret;
+
+	/*
+	 * Get file attributes for validation. We use vfs_getattr() to ensure
+	 * we have up-to-date file size information.
+	 */
+	ret = vfs_getattr_nosec(&file->f_path, &stat, STATX_SIZE, 
+			        AT_STATX_SYNC_AS_STAT);
+	if (ret) {
+		/*
+		 * If we can't get attributes, fall back to i_size_read()
+		 * which should work for most cases.
+		 */
+		loopsize = i_size_read(file->f_mapping->host);
+	} else {
+		/* Use the size from getattr for consistency */
+		loopsize = stat.size;
+	}
 
-	/* Compute loopsize in bytes */
-	loopsize = i_size_read(file->f_mapping->host);
 	if (offset > 0)
 		loopsize -= offset;
 	/* offset is beyond i_size, weird but possible */
-- 
2.43.5