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

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Message-ID: <20200804154451.GA948167@google.com>
Date:   Tue, 4 Aug 2020 15:44:51 +0000
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        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 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.