Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma_lock

[Rik van Riel <riel@redhat.com>]

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.

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