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Date: Sat, 02 Feb 2002 14:16:41 -0600
From: Stephen Lord <lord@sgi.com>
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To: Chris Mason <mason@suse.com>
CC: Andrea Arcangeli <andrea@suse.de>, Chris Wedgwood <cw@f00f.org>,
        Andrew Morton <akpm@zip.com.au>, Ricardo Galli <gallir@uib.es>,
        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: O_DIRECT fails in some kernel and FS
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Chris Mason wrote:

>
>On Saturday, February 02, 2002 08:54:38 PM +0100 Andrea Arcangeli <andrea@suse.de> wrote:
>
>>>Chris and I had initially decided to unpack the tails on file open
>>>if O_DIRECT is used, but it seems cleaner to add a 
>>>reiserfs_get_block_direct_io, and have it return -EINVAL if a read
>>>went to a tail.  writes that happen to a tail will trigger tail
>>>conversion.
>>>
>>This is a safe approch (no risk of corruption etc..). However to provide
>>the same semantics of the other filesystems it would be even better if
>>we could unpack the tail within reiserfs_get_block_direct_io rather than
>>returning -EINVAL, but ok, most apps should work fine anyways (and as
>>worse people can workaround the magic by remounting reiserfs with notail
>>before writing the data that will need to be handled later via
>>O_DIRECT).
>>
>
>In the normal case, O_DIRECT can't be done on a file with a tail. 
>
>The way I read generic_file_direct_IO, O_DIRECT is only done in 
>units that start block aligned, and continue for a block aligned 
>length.  So, this can never include a packed file tail.  
>
>We should only need to worry if i_size on the file is wrong, and allows a 
>read/write to a block aligned chunk on a file with a tail, which should
>only be legal in the expanding truncate case from older kernels.  The
>-EINVAL return should only happen in this (very unlikely) case.
>
>-chris
>
Can't you fall back to buffered I/O for the tail? OK it complicates the
code, probably a lot, but it keeps things sane from the user's point of
view.

Steve



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