Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 17 Mar 2009, Andrea Arcangeli wrote:
> 
> I don't think you can tackle this from add_to_swap because the page
> may be in the swapcache well before gup runs (gup(write=1) can map the
> swapcache as exclusive and read-write in the pte).

If it's in the swap cache, it should be mapped read-only, and gup(write=1) 
will do the COW break and un-swapcache it.

When can it be writably in the swap cache? The write-only thing is the one 
we use to invalidate stale swap cache entries, and when we mark those 
pages writable (in do_wp_page or do_swap_page) we always remove the page 
from the swap cache at the same time.

Or is there some other path I missed?

> My preference is still to keeps pages with elevated refcount pinned in
> the ptes like 2.6.7 did, that will allow do_wp_page to takeover only
> pages with page_count not elevated without risk of calling do_wp_page
> on any page under gup.

I agree that that would also work - and be even simpler. If done right, we 
can even avoid clearing the dirty bit (in page_mkclean()) for such pages, 
and now it works for _all_ pages, not just anonymous pages.

IOW, even if you had a shared mapping and were to GUP() those pages for 
writing, they'd _stay_ dirty until you free'd them - no need to re-dirty 
them in case somebody did IO on them. 

> Only worry I have now is how to compare count
> with mapcount when both can change under us if mapcount > 1, but if
> you meant page_mapcount in add_to_swap as I think, that logic in
> add_to_swap would have the same problem and so it needs a solution for
> doing a coherent/safe comparison too.

I don't think you can use just mapcount on its own - you have to compare 
it to page_count(). Otherwise perfectly normal (non-gup) pages will 
trigger, since that page count is the only thing that differs between the 
two cases.

			Linus

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