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

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: [REVIEW] Prevent direct I/O from mapping extents beyond eof
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Fri, 15 Aug 2008 18:09:58 -0400
Cc: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, akpm@xxxxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
In-reply-to: <48A50152.8020104@xxxxxxx>
References: <48A50152.8020104@xxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Mutt/1.5.18 (2008-05-17)
On Fri, Aug 15, 2008 at 02:08:50PM +1000, Lachlan McIlroy wrote:
> 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:

The change looks good to me, but I really think the direct I/O could
should never send down requests like this down to the filesystems.  akpm
and -fsdevel Cc'ed.

> --- 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)
---end quoted text---

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