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

From: Kalesh Singh <kaleshsingh@google.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jonathan Corbet <corbet@lwn.net>,
	kernel-team <kernel-team@android.com>,
	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>,
	linux-fsdevel@vger.kernel.org,
	Suren Baghdasaryan <surenb@google.com>,
	linux-media@vger.kernel.org
Subject: Re: [PATCH 2/2] dmabuf/tracing: Add dma-buf trace events
Date: Tue, 4 Aug 2020 15:44:51 +0000	[thread overview]
Message-ID: <20200804154451.GA948167@google.com> (raw)
In-Reply-To: <20200804010913.GA2096725@ZenIV.linux.org.uk>

On Tue, Aug 04, 2020 at 02:09:13AM +0100, Al Viro 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.
> > 
> > You are looking at the drastically wrong abstraction level.  Please, describe what
> > it is that you are trying to achieve.

Hi Al. Thank you for the comments. Ultimately what we need is to identify processes
that hold a file reference to the dma-buf. Unfortunately we can't use only
explicit dma_buf_get/dma_buf_put to track them because when an FD is being shared
between processes the file references are taken implicitly.

For example, on the sender side:
   unix_dgram_sendmsg -> send_scm -> __send_scm -> scm_fp_copy -> fget_raw
and on the receiver side:
   unix_dgram_recvmsg -> scm_recv -> scm_detach_fds -> __scm_install_fd -> get_file

I understand now that fd_install is not an appropriate abstraction level to track these.
Is there a more appropriate alternative where we could use to track these implicit file
references?

> _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.
> 
> 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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