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

[Yafang Shao <laoar.shao@gmail.com>]

2017-11-18 0:45 GMT+08:00 Roman Gushchin <guro@fb.com>:
> On Sat, Nov 18, 2017 at 12:20:40AM +0800, Yafang Shao wrote:
>> 2017-11-17 23:55 GMT+08:00 Roman Gushchin <guro@fb.com>:
>> > 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@gmail.com> 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,
>>
>> Yes. But that's irrelevant.
>>
>> > and using a particular limit value
>> > at a moment of tmpfs mounting doesn't provide any warranties further.
>> >
>>
>> I can not agree.
>> The default size of tmpfs is totalram / 2, the reason we do this is to
>> provide any warranties further IMHO.
>>
>> > Is there a reason why the userspace app which is mounting tmpfs can't
>> > set the size based on memory.limit?
>>
>> That's because of misuse.
>> The application should set size with "-o size=" when mount tmpfs, but
>> not all applications do this.
>> As we can't guarantee that all applications will do this, we should
>> give them a proper default value.
>
> The value you're suggesting is proper only if an app which is mounting
> tmpfs resides in the same memcg

Yes.
But maybe that's mostly used today?

> and the memory limit will not be adjusted
> significantly later.

There's a similar issue for physical memory adjusted by memory hotplug.
So what will happen if the physical memory adjusted significantly later ?

> Otherwise you can end up with a default value, which
> is worse than totalram/2, for instance, if tmpfs is mounted by some helper,
> which is located in a separate and very limited memcg.

That may happen.
Maybe we could improve the solution to handle this issue ?


Thanks
Yafang

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