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

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Mar 11, 2009 at 08:06:55PM +0100, Ingo Molnar wrote:
> Good - i saw the '(fast-)gup fix' qualifier and fast-gup is a 
> fresh feature. If the problem existed in earlier kernels too 
> then i guess it isnt urgent.

It always existed yes. The reason of the (fast-) qualifier is because
gup-fast made it harder to fix this in mainline (there is also a patch
floating around for 2.6.18 based kernels that is simpler thanks to
gup-fast not being there). The trouble of gup-fast is that doing the
check of page_count inside PT lock (or mmap_sem write mode like in
fork(), but ksm only takes mmap_sem in read mode and it relied on PT
lock only) wasn't enough anymore to be sure the page_count wouldn't
increase from under us just after we read it, because a gup-fast could
be running in another CPU without mmap_sem and without PT lock
taken. So fixing this on mainline has been a bit harder as I had to
prevent gup-fast to go ahead in the fast path, in a way that didn't
send IPIs to flush the smp-tlb before reading the page_count (so to
avoid sending IPIs for every anon page mapped writeable).

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