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SubjectRe: [-mm][PATCH 4/4] Add memrlimit controller accounting and control (v4)
On Thu, May 15, 2008 at 1:25 AM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> >
> > But the only *new* cases of taking the mmap_sem that this would
> > introduce would be:
> >
> > - on a failed vm limit charge
>
> Why a failed charge? Aren't we talking of moving all charge/uncharge
> under mmap_sem?
>

Sorry, I worded that wrongly - I meant "cleaning up a successful
charge after an expansion fails for other reasons"

I thought that all the charges and most of the uncharges were already
under mmap_sem, and it would just be a few of the cleanup paths that
needed to take it.

>
> > - when a task moves between two cgroups in the memrlimit hierarchy.
> >
>
> Yes, this would nest cgroup_mutex and mmap_sem. Not sure if that would
> be a bad side-effect.
>

I think it's already nested that way - e.g. the cpusets code can call
various migration functions (which take mmap_sem) while holding
cgroup_mutex.

>
> Refactor the code to try and use mmap_sem and see what I come up
> with. Basically use mmap_sem for all charge/uncharge operations as
> well use mmap_sem in read_mode in the move_task() and
> mm_owner_changed() callbacks. That should take care of the race
> conditions discussed, unless I missed something.

Sounds good.

Thanks,

Paul


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