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

From: Suren Baghdasaryan <surenb@google.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jonathan Corbet <corbet@lwn.net>,
	DRI mailing list <dri-devel@lists.freedesktop.org>,
	linux-doc@vger.kernel.org, Ioannis Ilkos <ilkos@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	linaro-mm-sig@lists.linaro.org,
	Hridya Valsaraju <hridya@google.com>,
	Ingo Molnar <mingo@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Kalesh Singh <kaleshsingh@google.com>,
	linux-fsdevel@vger.kernel.org,
	kernel-team <kernel-team@android.com>,
	linux-media@vger.kernel.org
Subject: Re: [PATCH 2/2] dmabuf/tracing: Add dma-buf trace events
Date: Mon, 3 Aug 2020 19:10:59 -0700	[thread overview]
Message-ID: <CAJuCfpE3fqCDfNmKphg6ZkHVb-B07_jvhquVgwgPHPi1CTupmw@mail.gmail.com> (raw)
In-Reply-To: <20200804010913.GA2096725@ZenIV.linux.org.uk>

On Mon, Aug 3, 2020 at 6:09 PM Al Viro <viro@zeniv.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.
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