xfs: clean up the initial read logic in xfs_readsb

The initial sb read is always for a device logical block size buffer.
The device logical block size is provided in the bt_logical_sectorsize in
struct buftarg, so use that instead of the confusingly named
xfs_getsize_buftarg buffer that reads it from the bdev.

Update the comments surrounding the code to better describe what is going
on.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Carlos Maiolino <[email protected]>

Author: Christoph Hellwig

fs/xfs/xfs_buf.h

@@ -375,7 +375,6 @@ extern void xfs_buftarg_wait(struct xfs_buftarg *);
 extern void xfs_buftarg_drain(struct xfs_buftarg *);
 int xfs_configure_buftarg(struct xfs_buftarg *btp, unsigned int sectorsize);
 
-#define xfs_getsize_buftarg(buftarg)	block_size((buftarg)->bt_bdev)
 #define xfs_readonly_buftarg(buftarg)	bdev_read_only((buftarg)->bt_bdev)
 
 int xfs_buf_reverify(struct xfs_buf *bp, const struct xfs_buf_ops *ops);

fs/xfs/xfs_mount.c

@@ -171,19 +171,16 @@ xfs_readsb(
 	ASSERT(mp->m_ddev_targp != NULL);
 
 	/*
-	 * For the initial read, we must guess at the sector
-	 * size based on the block device.  It's enough to
-	 * get the sb_sectsize out of the superblock and
-	 * then reread with the proper length.
-	 * We don't verify it yet, because it may not be complete.
+	 * In the first pass, use the device sector size to just read enough
+	 * of the superblock to extract the XFS sector size.
+	 *
+	 * The device sector size must be smaller than or equal to the XFS
+	 * sector size and thus we can always read the superblock.  Once we know
+	 * the XFS sector size, re-read it and run the buffer verifier.
 	 */
-	sector_size = xfs_getsize_buftarg(mp->m_ddev_targp);
+	sector_size = mp->m_ddev_targp->bt_logical_sectorsize;
 	buf_ops = NULL;
 
-	/*
-	 * Allocate a (locked) buffer to hold the superblock. This will be kept
-	 * around at all times to optimize access to the superblock.
-	 */
 reread:
 	error = xfs_buf_read_uncached(mp->m_ddev_targp, XFS_SB_DADDR,
 				      BTOBB(sector_size), &bp, buf_ops);
@@ -247,6 +244,10 @@ xfs_readsb(
 	/* no need to be quiet anymore, so reset the buf ops */
 	bp->b_ops = &xfs_sb_buf_ops;
 
+	/*
+	 * Keep a pointer of the sb buffer around instead of caching it in the
+	 * buffer cache because we access it frequently.
+	 */
 	mp->m_sb_bp = bp;
 	xfs_buf_unlock(bp);
 	return 0;