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

[Dave Chinner <david@fromorbit.com>]

On Tue, May 04, 2010 at 11:27:50AM -0400, Josef Bacik wrote:
> 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,

Yes, that looks like it would work to me. Might be worth a comment,
though.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com