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

From: Christoph Hellwig
Date: Fri Nov 21 2014 - 05:07:10 EST

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.
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