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Date: Sun, 03 Feb 2002 09:05:04 -0600
From: Stephen Lord <lord@sgi.com>
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To: Chris Wedgwood <cw@f00f.org>
CC: Jeff Garzik <garzik@havoc.gtf.org>, Chris Mason <mason@suse.com>,
        Andrea Arcangeli <andrea@suse.de>, 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 Wedgwood wrote:

>On Sun, Feb 03, 2002 at 07:40:57AM -0600, Stephen Lord wrote:
>
>    What we had were two flags, one which indicated use direct I/O,
>    and another which indicated return an error to user space rather
>    than go through buffers.  So lie to me and make it work, or don't
>    lie to me options I suppose.
>
>This seems way to complex in the case of reiserfs... you're only going
>to see tails for small files (typically under 16k) and for the tail
>part when less than a block.
>
>Since O_DIRECT much be blocked sized and block aligned, I'm not sure
>if this is a problem at present...
>

I agree is is not a big issue in this case - my interpretation of tails 
was the end
of any file could be packed, but if it is only small files.....

>
>
>    I suspect the reason XFS never did small files in the inode was
>    because of the problems with implementing mmap and O_DIRECT.
>
>How does IRIX deal with O_DIRECT read/writes of a mapped area?
>Invalidate them or just accept things as being incoherent?
>

They are invalidated at the start of the I/O, but page faults are not 
blocked
out for the duration of the I/O, so the coherency is weak. However, if an
application is doing a combination of mmapped and direct I/O to a file
at the same time, then it should generally have some form of user space
synchronization anyway. For an application doing its own synchronization
of different I/Os they are coherent.

>
>
>
>    --cw
>

Steve


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