Re: page_waitqueue() considered harmful

On Thu, Sep 29, 2016 at 10:55:44PM +1000, Nicholas Piggin wrote:

On Thu, 29 Sep 2016 10:01:30 +0200
Peter Zijlstra [off-list ref] wrote:

quoted

On Tue, Sep 27, 2016 at 09:54:12AM +0100, Mel Gorman wrote:

quoted

On Tue, Sep 27, 2016 at 09:30:55AM +0200, Peter Zijlstra wrote:
Simple is relative unless I drastically overcomplicated things and it
wouldn't be the first time. 64-bit only side-steps the page flag issue
as long as we can live with that.  

So one problem with the 64bit only pageflags is that they do eat space
from page-flags-layout, we do try and fit a bunch of other crap in
there, and at some point that all will not fit anymore and we'll revert
to worse.

I've no idea how far away from that we are for distro kernels. I suppose
they have fairly large NR_NODES and NR_CPUS.

I know it's not fashionable to care about them anymore, but it's sad if
32-bit architectures miss out fundamental optimisations like this because
we're out of page flags. It would also be sad to increase the size of
struct page because we're too lazy to reduce flags. There's some that
might be able to be removed.

I'm all for cleaning some of that up, but its been a long while since I
poked in that general area.

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