Date:   Tue, 31 Oct 2017 19:01:50 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Yang Shi <yang.s@...baba-inc.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-mm@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org
Subject: Re: [RFC PATCH] fs: fsnotify: account fsnotify metadata to kmemcg

On Tue, Oct 31, 2017 at 6:52 PM, Jan Kara <jack@...e.cz> wrote:
> On Tue 31-10-17 13:51:40, Amir Goldstein wrote:
>> On Tue, Oct 31, 2017 at 12:50 PM, Jan Kara <jack@...e.cz> wrote:
>> > On Sun 22-10-17 11:24:17, Amir Goldstein wrote:
>> >> But I think there is another problem, not introduced by your change, but could
>> >> be amplified because of it - when a non-permission event allocation fails, the
>> >> event is silently dropped, AFAICT, with no indication to listener.
>> >> That seems like a bug to me, because there is a perfectly safe way to deal with
>> >> event allocation failure - queue the overflow event.
>> >>
>> >> I am not going to be the one to determine if fixing this alleged bug is a
>> >> prerequisite for merging your patch, but I think enforcing memory limits on
>> >> event allocation could amplify that bug, so it should be fixed.
>> >>
>> >> The upside is that with both your accounting fix and ENOMEM = overlflow
>> >> fix, it going to be easy to write a test that verifies both of them:
>> >> - Run a listener in memcg with limited kmem and unlimited (or very
>> >> large) event queue
>> >> - Produce events inside memcg without listener reading them
>> >> - Read event and expect an OVERFLOW event
>> >>
>> >> This is a simple variant of LTP tests inotify05 and fanotify05.
>> >>
>> >> I realize that is user application behavior change and that documentation
>> >> implies that an OVERFLOW event is not expected when using
>> >> FAN_UNLIMITED_QUEUE, but IMO no one will come shouting
>> >> if we stop silently dropping events, so it is better to fix this and update
>> >> documentation.
>> >>
>> >> Attached a compile-tested patch to implement overflow on ENOMEM
>> >> Hope this helps to test your patch and then we can merge both, accompanied
>> >> with LTP tests for inotify and fanotify.
>> >>
>> >> Amir.
>> >
>> >> From 112ecd54045f14aff2c42622fabb4ffab9f0d8ff Mon Sep 17 00:00:00 2001
>> >> From: Amir Goldstein <amir73il@...il.com>
>> >> Date: Sun, 22 Oct 2017 11:13:10 +0300
>> >> Subject: [PATCH] fsnotify: queue an overflow event on failure to allocate
>> >>  event
>> >>
>> >> In low memory situations, non permissions events are silently dropped.
>> >> It is better to queue an OVERFLOW event in that case to let the listener
>> >> know about the lost event.
>> >>
>> >> With this change, an application can now get an FAN_Q_OVERFLOW event,
>> >> even if it used flag FAN_UNLIMITED_QUEUE on fanotify_init().
>> >>
>> >> Signed-off-by: Amir Goldstein <amir73il@...il.com>
>> >
>> > So I agree something like this is desirable but I'm uneasy about using
>> > {IN|FAN}_Q_OVERFLOW for this. Firstly, it is userspace visible change for
>> > FAN_UNLIMITED_QUEUE queues which could confuse applications as you properly
>> > note. Secondly, the event is similar to queue overflow but not quite the
>> > same (it is not that the application would be too slow in processing
>> > events, it is just that the system is in a problematic state overall). What
>> > are your thoughts on adding a new event flags like FAN_Q_LOSTEVENT or
>> > something like that? Probably the biggest downside there I see is that apps
>> > would have to learn to use it...
>> >
>>
>> Well, I can't say I like FAN_Q_LOSTEVENT, but I can't really think of
>> a better option. I guess apps that would want to provide better protection
>> against loosing event will have to opt-in with a new fanotify_init() flag.
>> OTOH, if apps opts-in for this feature, we can also report Q_OVERFLOW
>> and document that it *is* expected in OOM situation.
>>
>> If we have FAN_Q_LOSTEVENT, we can use it to handle both the case of
>> error to queue event (-ENOMEM) and the case of error on copy event to user
>> (e.g. -ENODEV), which is another case where we silently drop events
>> (in case buffer already contains good events).
>> In latter case, the error would be reported to user on event->fd.
>> In the former case, event->fd will also hold the error, as long as we can only
>> report -ENOMEM from this sort of error, because like overflow event, there
>> should probably be only one event of that sort in the queue.
>>
>> Another option for API name is {IN|FAN}_Q_ERR, which implies that event->fd
>> carries the error. And of course user can get an event with mask
>> FAN_Q_OVERFLOW|FAN_Q_ERR, where event->fd is -ENOMEM or
>> -EOVERFLOW and then there is no ambiguity between different kind of
>> queue overflows.
>
> I like this last option. I.e., userspace can opt in to get more detailed
> error notification. In that case we can report error (I think we can just
> reuse {IN|FAN}_Q_OVERFLOW for that) and store more detailed error
> description in wd/fd. Will you have time to implement something like that
> or should I put it to my todo list?
>

Won't be able to get to that for a while, so better add to todo list.

Thanks,
Amir.

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