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

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Message-ID: <20200804010913.GA2096725@ZenIV.linux.org.uk>
Date:   Tue, 4 Aug 2020 02:09:13 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Suren Baghdasaryan <surenb@...gle.com>
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 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.

_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.