On Mon, Sep 12, 2011 at 11:56:09AM +0200, Peter Zijlstra wrote:

On Mon, 2011-09-12 at 10:34 +0100, Mel Gorman wrote:

quoted

On Mon, Sep 12, 2011 at 11:04:45AM +0200, Peter Zijlstra wrote:

quoted

On Fri, 2011-09-09 at 09:36 -0400, Christoph Hellwig wrote:

quoted

The equivalent of ->direct_IO should be used for both reads and writes.

So the difference between DIO and swapIO is that swapIO needs the block
map pinned in memory.. So at the very least you'll need those
swap_{activate,deactivate} aops. The read/write-page thingies could
indeed be shared with DIO.

I'm travelling at the moment so it'll be later in the week when I investigate
properly but I agree swap_[de|a]ctivate are still necessary. NFS does not
need to pin a block map but it's still necessary for calling xs_set_memalloc.

Right.. but I think the hope was that we could replace the current swap
bmap hackery with this and simplify the normal swap bits. But yeah,
networked filesystems don't really bother with block maps on the client
side ;-)

I took a look at what was involved with doing the block lookups in
ext4. It's what led to patch 4 of this series because it was necessary that
the filesystem get the same information as the generic handler. It got a
bit messy but looked like it would have worked if I kept at it. I stopped
because I did nt see a major advantage with swap_writepage() looking up
the block map instead of having looked it up in advance with bmap() but
I could have missed something.

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