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

From: Yafang Shao
Date: Fri Nov 17 2017 - 21:29:58 EST

2017-11-18 1:49 GMT+08:00 Shakeel Butt <shakeelb@xxxxxxxxxx>:
> On Fri, Nov 17, 2017 at 9:41 AM, Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>> 2017-11-18 1:35 GMT+08:00 Shakeel Butt <shakeelb@xxxxxxxxxx>:
>>> On Fri, Nov 17, 2017 at 9:09 AM, Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>>>> 2017-11-18 0:45 GMT+08:00 Roman Gushchin <guro@xxxxxx>:
>>>>> On Sat, Nov 18, 2017 at 12:20:40AM +0800, Yafang Shao wrote:
>>>>>> 2017-11-17 23:55 GMT+08:00 Roman Gushchin <guro@xxxxxx>:
>>>>>> > 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,
>>>>>> 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 ?
>>> Let's backtrack, what is the actual concern? If a user/process inside
>>> a memcg is allocating pages for a file on a tmpfs mounted without size
>>> parameter, you want the OS to return ENOSPC (if allocation is done by
>>> write syscall) earlier to not cause the user/process's memcg to OOM.
>>> Is that right?
>> Right.
>>> First, there is no guarantee to not cause OOM by restricting tmpfs to
>>> half the size of memcg limit due to the presence of other memory
>>> charged to that memcg. The memcg can OOM even before the tmpfs hits
>>> its size.
>> Just guarantee that the OOM not caused by misuse of tmpfs.
>>> Second, the users who really care to avoid such scenario should just
>>> set the size parameter of tmpfs.
>> Of couse that is the best way.
>> But we can not ensue all applications will do it.
>> That's why I introduce a proper defalut value for them.
> I think we disagree on the how to get proper default value. Unless you
> can restrict that all the memory allocated for a tmpfs mount will be
> charged to a specific memcg, you should not just pick limit of the
> memcg of the process mounting the tmpfs to set the default of tmpfs
> mount. If you can restrict tmpfs charging to a specific memcg then the
> limit of that memcg should be used to set the default of the tmpfs
> mount. However this feature is not present in the upstream kernel at
> the moment (We have this feature in our local kernel and I am planning
> to upstream that).

That will be fine if you could upstream this feature ASAP :)