[XFS] Don't do I/O beyond eof when unreserving space

When unreserving space with boundaries that are not block aligned we round
up the start and round down the end boundaries and then use this function,
xfs_zero_remaining_bytes(), to zero the parts of the blocks that got
dropped during the rounding. The problem is we don't consider if these
blocks are beyond eof. Worse still is if we encounter delayed allocations
beyond eof we will try to use the magic delayed allocation block number as
a real block number. If the file size is ever extended to expose these
blocks then we'll go through xfs_zero_eof() to zero them anyway.

SGI-PV: 983683

SGI-Modid: xfs-linux-melb:xfs-kern:32055a

Signed-off-by: Lachlan McIlroy <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

fs/xfs/xfs_vnodeops.c

···
 /*
  * Zero file bytes between startoff and endoff inclusive.
  * The iolock is held exclusive and no blocks are buffered.
+ *
+ * This function is used by xfs_free_file_space() to zero
+ * partial blocks when the range to free is not block aligned.
+ * When unreserving space with boundaries that are not block
+ * aligned we round up the start and round down the end
+ * boundaries and then use this function to zero the parts of
+ * the blocks that got dropped during the rounding.
  */
 STATIC int
 xfs_zero_remaining_bytes(
···
 	xfs_mount_t		*mp = ip->i_mount;
 	int			nimap;
 	int			error = 0;
+
+	/*
+	 * Avoid doing I/O beyond eof - it's not necessary
+	 * since nothing can read beyond eof.  The space will
+	 * be zeroed when the file is extended anyway.
+	 */
+	if (startoff >= ip->i_size)
+		return 0;
+
+	if (endoff > ip->i_size)
+		endoff = ip->i_size;
 
 	bp = xfs_buf_get_noaddr(mp->m_sb.sb_blocksize,
 				XFS_IS_REALTIME_INODE(ip) ?