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

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

2017-11-18 1:35 GMT+08:00 Shakeel Butt <shakeelb@google.com>:
> On Fri, Nov 17, 2017 at 9:09 AM, Yafang Shao <laoar.shao@gmail.com> wrote:
>> 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 ?
>>
>>
>
> 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.


Thanks
Yafang

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