Re: [RFC PATCH 0/6] btrfs: implement swap file support

[Omar Sandoval <osandov@osandov.com>]

On Fri, Nov 21, 2014 at 02:06:57AM -0800, Christoph Hellwig wrote:
> On Tue, Nov 18, 2014 at 11:22:35PM -0800, Omar Sandoval wrote:
> > Here's a nice little bit of insanity I put together in that direction --
> > consider it a discussion point more than a patch. It does two things:
> > 
> > - Uses an ITER_BVEC iov_iter to do direct_IO for swap_readpage. This makes
> >   swap_readpage a synchronous operation, but I think that's the best we can do
> >   with the existing interface.
> 
> Note that ->read_iter for direct-io supports async I/O in general.  By
> resurrecting some of the older attempts to do in-kernel aio this could
> be made async easily.
> 
> > - Unless I'm missing something, there don't appear to be any instances of
> >   ITER_BVEC | READ in the kernel, so the dio path doesn't know not to dirty
> >   pages it gets that way. Dave Kleikamp and Ming Lei each previously submitted
> >   patches doing this as part of adding an aio_kernel interface. (The NFS direct
> >   I/O implementation doesn't know how to deal with these either, so this patch
> >   actually breaks the only existing user of this code path, but in the interest
> >   of keeping the patch short, I didn't try to fix it :)
> 
> Right, we'd need to look into.  Bonus points of allowing this as a zero
> copy read.
> 
> 
> Btw, in the long run I would much prefer killing of the current horrible
> swap using bmap path in favor of an enhanced direct I/O path.
Looks like I just raced on your email and sent a v2 of my patch series before
seeing this response :) I'll take a closer look at this tomorrow, thanks for
getting back to me.

-- 
Omar