Re: [PATCH v2 3/3] fs: fsnotify: account fsnotify metadata to kmemcg

[Shakeel Butt <shakeelb@google.com>]

On Thu, Feb 22, 2018 at 6:48 AM, Jan Kara <jack@suse.cz> wrote:
> On Thu 22-02-18 14:49:44, Michal Hocko wrote:
>> On Tue 20-02-18 19:01:01, Shakeel Butt wrote:
>> > A lot of memory can be consumed by the events generated for the huge or
>> > unlimited queues if there is either no or slow listener. This can cause
>> > system level memory pressure or OOMs. So, it's better to account the
>> > fsnotify kmem caches to the memcg of the listener.
>>
>> How much memory are we talking about here?
>
> 32 bytes per event (on 64-bit) which is small but the number of events is
> not limited in any way (if the creator uses a special flag and has
> CAP_SYS_ADMIN). In the thread [1] a guy from Alibaba wanted this feature so
> among cloud people there is apparently some demand to have a way to limit
> memory usage of such application...
>
>> > There are seven fsnotify kmem caches and among them allocations from
>> > dnotify_struct_cache, dnotify_mark_cache, fanotify_mark_cache and
>> > inotify_inode_mark_cachep happens in the context of syscall from the
>> > listener. So, SLAB_ACCOUNT is enough for these caches.
>> >
>> > The objects from fsnotify_mark_connector_cachep are not accounted as
>> > they are small compared to the notification mark or events and it is
>> > unclear whom to account connector to since it is shared by all events
>> > attached to the inode.
>> >
>> > The allocations from the event caches happen in the context of the event
>> > producer. For such caches we will need to remote charge the allocations
>> > to the listener's memcg. Thus we save the memcg reference in the
>> > fsnotify_group structure of the listener.
>>
>> Is it typical that the listener lives in a different memcg and if yes
>> then cannot this cause one memcg to OOM/DoS the one with the listener?
>
> We have been through these discussions already in [1] back in November :).
> I can understand the wish to limit memory usage of an application using
> unlimited fanotify queues. And yes, it may mean that it will be easier for
> an attacker to get it oom-killed (currently the malicious app would drive
> the whole system oom which will presumably take a bit more effort as there
> is more memory to consume). But then I expect this is what admin prefers
> when he limits memory usage of fanotify listener.
>

Just one clarification, currently the kernel does not trigger
oom-killer for allocations hitting memcg limit in the context of
syscalls but rather return an ENOMEM (after trying memcg reclaim). Jan
has already posted a patch to handle those ENOMEMs.

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