Re: remove zero_page (was Re: -mm merge plans for 2.6.24)



On Wed, 3 Oct 2007, Nick Piggin wrote:

I don't know if Linus actually disliked the patch itself, or disliked
my (maybe confusingly worded) rationale?

Yes. I'd happily accept the patch, but I'd want it clarified and made
obvious what the problem was - and it wasn't the zero page itself, it was
a regression in the VM that made it less palatable.

I also thought that there were potentially better solutions, namely to
simply avoid the VM regression, but I also acknowledge that they may not
be worth it - I just want them to be on the table.

In short: the real cost of the zero page was the reference counting on the
page that we do these days. For example, I really do believe that the
problem could fairly easily be fixed by simply not considering zero_page
to be a "vm_normal_page()". We already *do* have support for pages not
getting ref-counted (since we need it for other things), and I think that
zero_page very naturally falls into exactly that situation.

So in many ways, I would think that turning zero-page into a nonrefcounted
page (the same way we really do have to do for other things anyway) would
be the much more *direct* solution, and in many ways the obvious one.

HOWEVER - if people think that it's easier to remove zero_page, and want
to do it for other reasons, *AND* can hopefully even back up the claim
that it never matters with numbers (ie that the extra pagefaults just make
the whole zero-page thing pointless), then I'd certainly accept the patch.

I'd just want the patch *description* to then also be correct, and blame
the right situation, instead of blaming zero-page itself.

Linus
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