Date:   Tue, 13 Feb 2018 17:59:57 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Amir Goldstein <amir73il@...il.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 Tue, Feb 13, 2018 at 2:20 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
> On Tue, Feb 13, 2018 at 1:54 PM, Amir Goldstein <amir73il@...il.com> wrote:
>> On Tue, Feb 13, 2018 at 11:10 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
>>> On Mon, Feb 12, 2018 at 10:30 PM, Amir Goldstein <amir73il@...il.com> wrote:
>>>> 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.
>>>>
>>>
>>> I thought the reason to opt-in "charge memory to listener" was the
>>> risk of oom-killing the listener but it is now clear that there will
>>> be no oom-kills on memcg hitting its limit (no oom-killing listener
>>> risk). In my (not so strong) opinion we should only opt-in for
>>> receiving the {FAN|IN}_Q_OVERFLOW event on ENOMEM but always charge
>>> the memory for events to the listener's memcg if kmem accounting is
>>> enabled.
>>>
>>
>> I agree that charging listener's memcg is preferred, but it is still a change
>> of behavior, because if attacker can allocate memory from listener's memcg,
>> then attacker can force overflow and hide the traces of its own filesystem
>> operations.
>>
>
> ACK.
>
>>>> Something like FAN_GROUP_QUEUE  (better name is welcome)
>>>> which is mutually exclusive (?) with FAN_UNLIMITED_QUEUE.
>>>>
>
> How about FAN_CHARGE_MEMCG?
>

Also should there be a similar flag for inotify_init1() as well?

>>>
>>> There is no need to make them mutually exclusive. One should be able
>>> to request an unlimited queue limited by available memory on system
>>> (with no kmem charging) or limited by limit of the listener's memcg
>>> (with kmem charging).
>>
>> OK.
>>
>>>
>>>> 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?
>>>>
>>>
>>> Are there any error which are not related to queue overflows? I see
>>> the mention of ENODEV and EOVERFLOW in the discussion. If there are
>>> such errors and might be interesting to the listener then we should
>>> have 2 independent APIs.
>>>
>>
>> These are indeed 2 different use cases.
>> A Q_OVERFLOW event is only expected one of ENOMEM or
>> EOVERFLOW in event->fd, but other events (like open of special device
>> file) can have ENODEV in event->fd.
>>
>> But I am not convinced that those require 2 independent APIs.
>> Specifying FAN_Q_ERR means that the user expects to reads errors
>> from event->fd.
>>
>
> Can you please explain what you mean by 2 independent APIs? I thought
> "no independent APIs" means FAN_Q_ERR can only be used with
> FAN_Q_OVERFLOW and without FAN_Q_OVERFLOW, FAN_Q_ERR is ignored. Is
> that right or I misunderstood?
>
>>>> 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.
>>>>
>>>
>>> I think there is no need for imposing CAP_SYS_ADMIN for requesting to
>>> charge self for the event memory.
>>>
>>
>> Certainly. The question is whether the flag combination
>> FAN_GROUP_QUEUE|FAN_UNLIMITED_QUEUE could relax the
>> CAP_SYS_ADMIN requirement that is imposed by FAN_UNLIMITED_QUEUE
>> by itself.
>>
>
> Oh, I agree with relaxing CAP_SYS_ADMIN requirement if both flags are given.
>
>> Note that FAN_UNLIMITED_MARKS cannot relax CAP_SYS_ADMIN
>> even though marks are already accounted to listener memcg. This is because
>> most of the memory consumption in this case comes from marks pinning the
>> watched inodes to cache and not from the marks themselves.
>>
>
> thanks,
> Shakeel

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