Re: [RFC PATCH -v3] take all anon_vma locks in anon_vma_lock

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Apr 29, 2010 at 11:10 AM, Rik van Riel <riel@redhat.com> wrote:
> On 04/28/2010 08:28 PM, Minchan Kim wrote:
>>
>> On Thu, Apr 29, 2010 at 5:57 AM, Rik van Riel<riel@redhat.com>  wrote:
>>>
>>> Take all the locks for all the anon_vmas in anon_vma_lock, this properly
>>> excludes migration and the transparent hugepage code from VMA changes
>>> done
>>> by mmap/munmap/mprotect/expand_stack/etc...
>>>
>>> Unfortunately, this requires adding a new lock (mm->anon_vma_chain_lock),
>>> otherwise we have an unavoidable lock ordering conflict.  This changes
>>> the
>>> locking rules for the "same_vma" list to be either mm->mmap_sem for
>>> write,
>>> or mm->mmap_sem for read plus the new mm->anon_vma_chain lock.  This
>>> limits
>>> the place where the new lock is taken to 2 locations - anon_vma_prepare
>>> and
>>> expand_downwards.
>>>
>>> Document the locking rules for the same_vma list in the anon_vma_chain
>>> and
>>> remove the anon_vma_lock call from expand_upwards, which does not need
>>> it.
>>>
>>> Signed-off-by: Rik van Riel<riel@redhat.com>
>>
>> This patch makes things simple. So I like this.
>> Actually, I wanted this all-at-once locks approach.
>> But I was worried about that how the patch affects AIM 7 workload
>> which is cause of anon_vma_chain about scalability by Rik.
>> But now Rik himself is sending the patch. So I assume the patch
>> couldn't decrease scalability of the workload heavily.
>
> The thing is, the number of anon_vmas attached to a VMA is
> small (depth of the tree, so for apache or aim the typical
> depth is 2). This N is between 1 and 3.
>
> The problem we had originally is the _width_ of the tree,
> where every sibling process was attached to the same anon_vma
> and the rmap code had to walk the page tables of all the
> processes, for every privately owned page in each child process.
> For large server workloads, this N is between a few hundred and
> a few thousand.
>
> What matters most at this point is correctness - we need to be
> able to exclude rmap walks when messing with a VMA in any way
> that breaks lookups, because rmap walks for page migration and
> hugepage conversion have to be 100% reliable.
>
> That is not a constraint I had in mind with the original
> anon_vma changes, so the code needs to be fixed up now...

Yes. I understand it.

When you tried anon_vma_chain patches as I pointed out, what I have a
concern is parent's vma not child's one.
The vma of parent still has N anon_vma.
AFAIR, you said it's trade-off and would be good than old at least.
I agreed.  But I just want to remind you because this makes worse.  :)
The corner case is that we have to hold locks of N.

Do I miss something?
Really, Can't we ignore that case latency although this happen infrequently?
I am not against this patch. I just want to listen your opinion.

-- 
Kind regards,
Minchan Kim

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