Re: [RFC] Making memcg track ownership per address_space or anon_vma

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Thu, Feb 12, 2015 at 12:46 AM, Tejun Heo <tj@kernel.org> wrote:
> Hello,
>
> On Thu, Feb 12, 2015 at 12:22:34AM +0300, Konstantin Khlebnikov wrote:
>> > Yeah, available memory to the matching memcg and the number of dirty
>> > pages in it.  It's gonna work the same way as the global case just
>> > scoped to the cgroup.
>>
>> That might be a problem: all dirty pages accounted to cgroup must be
>> reachable for its own personal writeback or balanace-drity-pages will be
>> unable to satisfy memcg dirty memory thresholds. I've done accounting
>
> Yeah, it would.  Why wouldn't it?

How do you plan to do per-memcg/blkcg writeback for balance-dirty-pages?
Or you're thinking only about separating writeback flow into blkio cgroups
without actual inode filtering? I mean delaying inode writeback and keeping
dirty pages as long as possible if their cgroups are far from threshold.

>
>> for per-inode owner, but there is another option: shared inodes might be
>> handled differently and will be available for all (or related) cgroup
>> writebacks.
>
> I'm not following you at all.  The only reason this scheme can work is
> because we exclude persistent shared write cases.  As the whole thing
> is based on that assumption, special casing shared inodes doesn't make
> any sense.  Doing things like allowing all cgroups to write shared
> inodes without getting memcg on-board almost immediately breaks
> pressure propagation while making shared writes a lot more attractive
> and increasing implementation complexity substantially.  Am I missing
> something?
>
>> Another side is that reclaimer now (mosly?) never trigger pageout.
>> Memcg reclaimer should do something if it finds shared dirty page:
>> either move it into right cgroup or make that inode reachable for
>> memcg writeback. I've send patch which marks shared dirty inodes
>> with flag I_DIRTY_SHARED or so.
>
> It *might* make sense for memcg to drop pages being dirtied which
> don't match the currently associated blkcg of the inode; however,
> again, as we're basically declaring that shared writes aren't
> supported, I'm skeptical about the usefulness.
>
> Thanks.
>
> --
> tejun