Re: [PATCH] fs, mm: account filp and names caches to kmemcg

[Shakeel Butt <shakeelb@google.com>]

On Sun, Oct 8, 2017 at 11:24 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Fri 06-10-17 12:33:03, Shakeel Butt wrote:
>> >>       names_cachep = kmem_cache_create("names_cache", PATH_MAX, 0,
>> >> -                     SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
>> >> +                     SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL);
>> >
>> > I might be wrong but isn't name cache only holding temporary objects
>> > used for path resolution which are not stored anywhere?
>> >
>>
>> Even though they're temporary, many containers can together use a
>> significant amount of transient uncharged memory. We've seen machines
>> with 100s of MiBs in names_cache.
>
> Yes that might be possible but are we prepared for random ENOMEM from
> vfs calls which need to allocate a temporary name?
>

I looked at all the syscalls which invoke allocations from
'names_cache' and tried to narrow down whose man page does not mention
that they can return ENOMEM. I found couple of syscalls like
truncate(), readdir() & getdents() which does not mention that they
can return ENOMEM but this patch will make them return ENOMEM.

>>
>> >>       filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
>> >> -                     SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
>> >> +                     SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT, NULL);
>> >>       percpu_counter_init(&nr_files, 0, GFP_KERNEL);
>> >>  }
>> >
>> > Don't we have a limit for the maximum number of open files?
>> >
>>
>> Yes, there is a system limit of maximum number of open files. However
>> this limit is shared between different users on the system and one
>> user can hog this resource. To cater that, we set the maximum limit
>> very high and let the memory limit of each user limit the number of
>> files they can open.
>
> Similarly here. Are all syscalls allocating a fd prepared to return
> ENOMEM?

For filp, I found _sysctl(). However the man page says not to use it.

On Tue, Oct 10, 2017 at 2:10 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 09-10-17 20:17:54, Michal Hocko wrote:
>> the primary concern for this patch was whether we really need/want to
>> charge short therm objects which do not outlive a single syscall.
>
> Let me expand on this some more. What is the benefit of kmem accounting
> of such an object? It cannot stop any runaway as a syscall lifetime
> allocations are bound to number of processes which we kind of contain by
> other means.

We can contain by limited the number of processes or thread but for us
applications having thousands of threads is very common. So, limiting
the number of threads/processes will not work.

> If we do account then we put a memory pressure due to
> something that cannot be reclaimed by no means. Even the memcg OOM
> killer would simply kick a single path while there might be others
> to consume the same type of memory.
>
> So what is the actual point in accounting these? Does it help to contain
> any workload better? What kind of workload?
>

I think the benefits will be isolation and more accurate billing. As I
have said before we have observed 100s of MiBs in names_cache on many
machines and cumulative amount is not something we can ignore as just
memory overhead.

> Or am I completely wrong and name objects can outlive a syscall
> considerably?
>

No, I didn't find any instance of the name objects outliving the syscall.

Anyways, we can discuss more on names_cache, do you have any objection
regarding charging filp?

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