[XFS] Don't wait for pending I/Os when purging blocks beyond eof.

On last close of a file we purge blocks beyond eof. The same code is used
when we truncate the file size down. In this case we need to wait for any
pending I/Os for dirty pages beyond the new eof. For the last close case
we are not changing the file size and therefore do not need to wait for
any I/Os to complete. This fixes a performance bottleneck where writes
into the page cache and cache flushes can become mutually exclusive.

SGI-PV: 964002
SGI-Modid: xfs-linux-melb:xfs-kern:30220a
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Peter Leckie <pleckie@sgi.com>

fs/xfs/xfs_inode.c

@@ -1459,8 +1459,10 @@ xfs_itruncate_start(
 	mp = ip->i_mount;
 	vp = XFS_ITOV(ip);
 
-	vn_iowait(ip);  /* wait for the completion of any pending DIOs */
-	
+	/* wait for the completion of any pending DIOs */
+	if (new_size < ip->i_size)
+		vn_iowait(ip);
+
 	/*
 	 * Call toss_pages or flushinval_pages to get rid of pages
 	 * overlapping the region being removed.  We have to use