Message-ID: <48A50152.8020104@sgi.com>
Date: Fri, 15 Aug 2008 14:08:50 +1000
From: Lachlan McIlroy <lachlan@sgi.com>
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To: xfs-dev <xfs-dev@sgi.com>, xfs-oss <xfs@oss.sgi.com>
Subject: [REVIEW] Prevent direct I/O from mapping extents beyond eof
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With the help from some tracing I found that we try to map extents beyond
eof when doing a direct I/O read.  It appears that the way to inform the
generic direct I/O path (ie do_direct_IO()) that we have breached eof is
to return an unmapped buffer from xfs_get_blocks_direct().  This will cause
do_direct_IO() to jump to the hole handling code where is will check for
eof and then abort.

This problem was found because a direct I/O read was trying to map beyond
eof and was encountering delayed allocations.  The delayed allocations beyond
eof are speculative allocations and they didn't get converted when the direct
I/O flushed the file because there was only enough space in the current AG
to convert and write out the dirty pages within eof.  Note that
xfs_iomap_write_allocate() wont necessarily convert all the delayed allocation
passed to it - it will return after allocating the first extent - so if the
delayed allocation extends beyond eof then it will stay that way.

This change will detect a direct I/O read beyond eof:

--- a/fs/xfs/linux-2.6/xfs_aops.c	2008-08-15 13:30:03.000000000 +1000
+++ b/fs/xfs/linux-2.6/xfs_aops.c	2008-08-11 16:51:07.000000000 +1000
@@ -1338,6 +1338,10 @@ __xfs_get_blocks(
 	offset = (xfs_off_t)iblock << inode->i_blkbits;
 	ASSERT(bh_result->b_size >= (1 << inode->i_blkbits));
 	size = bh_result->b_size;
+
+	if (!create && direct && offset >= i_size_read(inode))
+		return 0;
+
 	error = xfs_iomap(XFS_I(inode), offset, size,
 			     create ? flags : BMAPI_READ, &iomap, &niomap);
 	if (error)