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[REVIEW] Prevent direct I/O from mapping extents beyond eof

To: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Subject: [REVIEW] Prevent direct I/O from mapping extents beyond eof
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Fri, 15 Aug 2008 14:08:50 +1000
Reply-to: lachlan@xxxxxxx
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Thunderbird 2.0.0.16 (X11/20080707)
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)


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