linux-kernel - Re: [PATCH 2/2] dmabuf/tracing: Add dma-buf trace events

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Date:   Mon, 3 Aug 2020 19:10:59 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, linux-media@...r.kernel.org,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        linaro-mm-sig@...ts.linaro.org, linux-fsdevel@...r.kernel.org,
        Hridya Valsaraju <hridya@...gle.com>,
        Ioannis Ilkos <ilkos@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 2/2] dmabuf/tracing: Add dma-buf trace events

On Mon, Aug 3, 2020 at 6:09 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Mon, Aug 03, 2020 at 11:28:31PM +0100, Al Viro wrote:
>
> > IOW, what the hell is that horror for?  You do realize, for example, that there's
> > such thing as dup(), right?  And dup2() as well.  And while we are at it, how
> > do you keep track of removals, considering the fact that you can stick a file
> > reference into SCM_RIGHTS datagram sent to yourself, close descriptors and an hour
> > later pick that datagram, suddenly getting descriptor back?
> >
> > Besides, "I have no descriptors left" != "I can't be currently sitting in the middle
> > of syscall on that sucker"; close() does *NOT* terminate ongoing operations.

Thanks for your feedback, Al. I see your points and sorry for not
realizing these shortcomings.

> >
> > You are looking at the drastically wrong abstraction level.  Please, describe what
> > it is that you are trying to achieve.
>
> _IF_ it's "who keeps a particularly long-lived sucker pinned", I would suggest
> fuser(1) run when you detect that kind of long-lived dmabuf.  With events generated
> by their constructors and destructors, and detection of longevity done based on
> that.

That is the intention here. IIUC fuser(1) would require root access to
collect this information from a process other than the caller. Ideally
what we would like to have is a non-root process with specific
capabilities (in our case a process that can access BPF maps) to be
able to obtain the information on dma-buf users.
However, it might make more sense to track dma-buf usage from
dma_buf_getfile, dma_buf_get and dma_buf_put since these calls are the
ones that affect file refcount. Will dig some more into this.
Thanks for your time and sorry for not thinking it through beforehand.

>
> But that's only a semi-blind guess at the things you are trying to achieve; please,
> describe what it really is.