loop: take the file system minimum dio alignment into account

The loop driver currently uses the logical block size of the underlying
bdev as the lower bound of the loop device block size.  While this works
for many cases, it fails for file systems made up of multiple devices
with different logical block sizes (e.g. XFS with a RT device that has a
larger logical block size), or when the file systems doesn't support
direct I/O writes at the sector size granularity (e.g. because it does
out of place writes with a file system block size larger than the sector
size).

Fix this by querying the minimum direct I/O alignment from statx when
available.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250131120120.1315125-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -54,7 +54,8 @@ struct loop_device {
 	int		lo_flags;
 	char		lo_file_name[LO_NAME_SIZE];
 
-	struct file *	lo_backing_file;
+	struct file	*lo_backing_file;
+	unsigned int	lo_min_dio_size;
 	struct block_device *lo_device;
 
 	gfp_t		old_gfp_mask;
@@ -169,29 +170,14 @@ static loff_t get_loop_size(struct loop_device *lo, struct file *file)
  * of backing device, and the logical block size of loop is bigger than that of
  * the backing device.
  */
-static bool lo_bdev_can_use_dio(struct loop_device *lo,
-		struct block_device *backing_bdev)
-{
-	unsigned int sb_bsize = bdev_logical_block_size(backing_bdev);
-
-	if (queue_logical_block_size(lo->lo_queue) < sb_bsize)
-		return false;
-	if (lo->lo_offset & (sb_bsize - 1))
-		return false;
-	return true;
-}
-
 static bool lo_can_use_dio(struct loop_device *lo)
 {
-	struct inode *inode = lo->lo_backing_file->f_mapping->host;
-
 	if (!(lo->lo_backing_file->f_mode & FMODE_CAN_ODIRECT))
 		return false;
-
-	if (S_ISBLK(inode->i_mode))
-		return lo_bdev_can_use_dio(lo, I_BDEV(inode));
-	if (inode->i_sb->s_bdev)
-		return lo_bdev_can_use_dio(lo, inode->i_sb->s_bdev);
+	if (queue_logical_block_size(lo->lo_queue) < lo->lo_min_dio_size)
+		return false;
+	if (lo->lo_offset & (lo->lo_min_dio_size - 1))
+		return false;
 	return true;
 }
 
@@ -539,6 +525,28 @@ static void loop_reread_partitions(struct loop_device *lo)
 			__func__, lo->lo_number, lo->lo_file_name, rc);
 }
 
+static unsigned int loop_query_min_dio_size(struct loop_device *lo)
+{
+	struct file *file = lo->lo_backing_file;
+	struct block_device *sb_bdev = file->f_mapping->host->i_sb->s_bdev;
+	struct kstat st;
+
+	/*
+	 * Use the minimal dio alignment of the file system if provided.
+	 */
+	if (!vfs_getattr(&file->f_path, &st, STATX_DIOALIGN, 0) &&
+	    (st.result_mask & STATX_DIOALIGN))
+		return st.dio_offset_align;
+
+	/*
+	 * In a perfect world this wouldn't be needed, but as of Linux 6.13 only
+	 * a handful of file systems support the STATX_DIOALIGN flag.
+	 */
+	if (sb_bdev)
+		return bdev_logical_block_size(sb_bdev);
+	return SECTOR_SIZE;
+}
+
 static inline int is_loop_device(struct file *file)
 {
 	struct inode *i = file->f_mapping->host;
@@ -579,6 +587,7 @@ static void loop_assign_backing_file(struct loop_device *lo, struct file *file)
 			lo->old_gfp_mask & ~(__GFP_IO | __GFP_FS));
 	if (lo->lo_backing_file->f_flags & O_DIRECT)
 		lo->lo_flags |= LO_FLAGS_DIRECT_IO;
+	lo->lo_min_dio_size = loop_query_min_dio_size(lo);
 }
 
 /*
@@ -976,12 +985,11 @@ loop_set_status_from_info(struct loop_device *lo,
 	return 0;
 }
 
-static unsigned int loop_default_blocksize(struct loop_device *lo,
-		struct block_device *backing_bdev)
+static unsigned int loop_default_blocksize(struct loop_device *lo)
 {
-	/* In case of direct I/O, match underlying block size */
-	if ((lo->lo_flags & LO_FLAGS_DIRECT_IO) && backing_bdev)
-		return bdev_logical_block_size(backing_bdev);
+	/* In case of direct I/O, match underlying minimum I/O size */
+	if (lo->lo_flags & LO_FLAGS_DIRECT_IO)
+		return lo->lo_min_dio_size;
 	return SECTOR_SIZE;
 }
 
@@ -999,7 +1007,7 @@ static void loop_update_limits(struct loop_device *lo, struct queue_limits *lim,
 		backing_bdev = inode->i_sb->s_bdev;
 
 	if (!bsize)
-		bsize = loop_default_blocksize(lo, backing_bdev);
+		bsize = loop_default_blocksize(lo);
 
 	loop_get_discard_config(lo, &granularity, &max_discard_sectors);