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

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

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...

I suspect that taking one or two extra spinlocks in the code
paths changed by this patch (mmap/munmap/...) is going to make
a difference at all, since all of those paths are pretty
infrequently taken.

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