Messages in this thread | | | Date | Mon, 3 May 2010 11:19:46 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma_lock |
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On Mon, 3 May 2010, Rik van Riel wrote: > > One problem is that we cannot find the VMAs (multiple) from > the page, except by walking the anon_vma_chain.same_anon_vma > list. At the very least, that list requires locking, done > by the anon_vma.lock.
But that's exactly what we do in rmap_walk() anyway.
But yes, I can well imagine that in other cases we only do the one anon_vma. I didn't check who used the lock.
So if we do want to keep the lock in the anon_vma, I would just suggest that instead of making "normal" users do lots of locking, make the rmap_walk side one.
> A forkbomb could definately end up getting slowed down by > this patch. Is there any real workload out there that just > forks deeper and deeper from the parent process, without > calling exec() after a generation or two?
Heh. AIM7. Wasn't that why we merged the multiple anon_vma's in the first place?
> > So again, my gut feel is that if the lock just were in the vma itself, > > then the "normal" users would have just one natural lock, while the > > special case users (rmap_walk_anon) would have to lock each vma it > > traverses. That would seem to be the more natural way to lock things. > > However ... there's still the issue of page_lock_anon_vma > in try_to_unmap_anon.
Do we care?
We've not locked them all there, and we've historically not cares about the rmap list being "perfect", have we?
So I _think_ it's just the migration case (and apparently potentially the hugepage case) that wants _exact_ information. Which is why I suggest the onus of the extra locking should be on _them_, not on the regular code.
I dunno. Again, my objections to the patches are really based more on a gut feel of "that can't be the right thing to do" than anything else.
We have _extremely_ few places that walk lists to lock things. And they are never "normal" code. Things like that magic "mm_take_all_locks()", for example. That is why I then react with "that can't be right" to patches like this.
Linus
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