linux-kernel - Re: [PATCH 1/3] fs: allow short direct-io reads to be completed via buffered IO

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Date:	Tue, 4 May 2010 11:27:50 -0400
From:	Josef Bacik <josef@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Josef Bacik <josef@...hat.com>, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...r.kernel.org
Subject: Re: [PATCH 1/3] fs: allow short direct-io reads to be completed
	via buffered IO

On Tue, May 04, 2010 at 10:14:18AM +1000, Dave Chinner wrote:
> On Mon, May 03, 2010 at 01:27:02PM -0400, Josef Bacik wrote:
> > This is similar to what already happens in the write case.  If we have a short
> > read while doing O_DIRECT, instead of just returning, fallthrough and try to
> > read the rest via buffered IO.  BTRFS needs this because if we encounter a
> > compressed or inline extent during DIO, we need to fallback on buffered.  If the
> > extent is compressed we need to read the entire thing into memory and
> > de-compress it into the users pages.  I have tested this with fsx and everything
> > works great.  Thanks,
> 
> Won't this mean that any direct IO read that spans EOF  (i.e. get a
> short read) now attempt a buffered IO (that will fail) before returning?
> 

Hmm yeah you are right.  What would be an acceptable way to avoid this, do a

if (retval || !count || ppos >= i_size_read(inode))
	goto out;

type thing?  Thanks,

Josef
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