Messages in this thread | | | Date | Mon, 03 May 2010 14:38:56 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma_lock |
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On 05/03/2010 02:19 PM, Linus Torvalds wrote: > 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.
Mel's original patch adds trylock & retry all code to rmap_walk and a few other places:
http://lkml.org/lkml/2010/4/26/321
I submitted my patch 1/2 as an alternative, because these repeated trylocks are pretty complex and easy to accidentally break when changes to other VM code are made.
>> 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?
AIM7, like sendmail, apache or postgresql, is only 2 deep.
>>> 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?
Well, try_to_unmap_anon walks just one page, and has the anon_vma for that page locked.
Having said that, for pageout we do indeed not care about getting it perfect.
> 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.
It's a matter of cost vs complexity. IMHO the locking changes in the lowest overhead patches (Mel's) are quite complex and could end up being hard to maintain in the future. I wanted to introduce something a little simpler, with hopefully minimal overhead.
But hey, that's just my opinion - what matters is that the bug gets fixed somehow. If you prefer the more complex but slightly lower overhead patches from Mel, that's fine too.
-- All rights reversed
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