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 11:55 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> 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.

me/ slaps self.

It's about height of tree and I can't imagine high height of
scenario.(fork->fork->fork->...->fork)
So as Rik pointed out, It's a not big overhead about latency latency,
at least. I think.

I supports this approach.

-- 
Kind regards,
Minchan Kim

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