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


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