linux-kernel - Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

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Date:   Fri, 19 Aug 2022 15:44:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     SeongJae Park <sj@...nel.org>
Cc:     gregkh@...uxfoundation.org, badari.pulavarty@...el.com,
        damon@...ts.linux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory
 creation

On Fri, 19 Aug 2022 21:16:31 +0000 SeongJae Park <sj@...nel.org> wrote:

> > It would be simpler (and less racy) to check the debugfs_create_dir()
> > return value for IS_ERR()?
> 
> I was merely following Greg's previous advice for ignoring the return value[1]
> of the function, but I might misunderstanding his intention, so CC-ing Greg.
> Greg, may I ask your opinion?
> 
> [1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/

Thing is, the correct functioning of the debugfs interfaces is utterly
critical to damon.  And that's apart from these memory leak and
oops-we-killed-damon issues.

So damon simply cannot ignore the state of its debugfs interfaces and
keep going along - because if something goes wrong at the debugfs
layer, damon is dead and useless and the machine needs a reboot.

Perhaps this means that damon should not be using debugfs for its
interfaces at all.  Or it means that the debugfs interfaces are
misdesigned.  I go with the latter, which, alas, also affirms the
former.

>From a quick scan it appears that a significant minority (20%?) of
drivers are checking the debugfs_create_dir() return value.