Re: [PATCH] mm/shmem: set default tmpfs size according to memcg limit

From: Roman Gushchin
Date: Fri Nov 17 2017 - 10:56:21 EST

On Thu, Nov 16, 2017 at 08:43:17PM -0800, Shakeel Butt wrote:
> On Thu, Nov 16, 2017 at 7:09 PM, Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> > Currently the default tmpfs size is totalram_pages / 2 if mount tmpfs
> > without "-o size=XXX".
> > When we mount tmpfs in a container(i.e. docker), it is also
> > totalram_pages / 2 regardless of the memory limit on this container.
> > That may easily cause OOM if tmpfs occupied too much memory when swap is
> > off.
> > So when we mount tmpfs in a memcg, the default size should be limited by
> > the memcg memory.limit.
> >
> The pages of the tmpfs files are charged to the memcg of allocators
> which can be in memcg different from the memcg in which the mount
> operation happened. So, tying the size of a tmpfs mount where it was
> mounted does not make much sense.

Also, memory limit is adjustable, and using a particular limit value
at a moment of tmpfs mounting doesn't provide any warranties further.

Is there a reason why the userspace app which is mounting tmpfs can't
set the size based on memory.limit?