linux-kernel - Re: [PATCH v2] fs: fsnotify: account fsnotify metadata to kmemcg

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Message-ID: <CAOQ4uxjgKUFJ_uhyrQdcTs1FzcN6JrR_JpPc9QBrGJEU+cf65w@mail.gmail.com>
Date:   Tue, 13 Feb 2018 08:30:27 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Jan Kara <jack@...e.cz>, Yang Shi <yang.s@...baba-inc.com>,
        Michal Hocko <mhocko@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org
Subject: Re: [PATCH v2] fs: fsnotify: account fsnotify metadata to kmemcg

On Thu, Jan 25, 2018 at 10:36 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Thu, Jan 25, 2018 at 10:20 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
>> On Wed, Jan 24, 2018 at 11:51 PM, Amir Goldstein <amir73il@...il.com> wrote:
>>>
>>> There is a nicer alternative, instead of failing the file access,
>>> an overflow event can be queued. I sent a patch for that and Jan
>>> agreed to the concept, but thought we should let user opt-in for this
>>> change:
>>> https://marc.info/?l=linux-fsdevel&m=150944704716447&w=2
>>>
>>> So IMO, if user opts-in for OVERFLOW instead of ENOMEM,
>>> charging the listener memcg would be non controversial.
>>> Otherwise, I cannot say that starting to charge the listener memgc
>>> for events won't break any application.
>>>
>>

Shakeel, Jan,

Reviving this thread and adding linux-api, because I think it is important to
agree on the API before patches.

The last message on the thread you referenced suggest an API change
for opting in for Q_OVERFLOW on ENOMEM:
https://marc.info/?l=linux-api&m=150946878623441&w=2

However, the suggested API change in in fanotify_mark() syscall and
this is not the time when fsnotify_group is initialized.
I believe for opting-in to accounting events for listener, you
will need to add an opt-in flag for the fanotify_init() syscall.

Something like FAN_GROUP_QUEUE  (better name is welcome)
which is mutually exclusive (?) with FAN_UNLIMITED_QUEUE.

The question is, do we need the user to also explicitly opt-in for
Q_OVERFLOW on ENOMEM with FAN_Q_ERR mark mask?
Should these 2 new APIs be coupled or independent?

Another question is whether FAN_GROUP_QUEUE may require
less than CAP_SYS_ADMIN? Of course for now, this is only a
semantic change, because fanotify_init() requires CAP_SYS_ADMIN
but as the documentation suggests, this may be relaxed in the future.

Thought?

Amir.