linux-kernel - Re: [PATCH 13/14] d_path: prepend_path() is unlikely to return non-zero

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Date:   Mon, 28 Jun 2021 04:14:03 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Justin He <Justin.He@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 13/14] d_path: prepend_path() is unlikely to return
 non-zero

On Mon, Jun 28, 2021 at 03:28:19AM +0000, Justin He wrote:

> > On which loads?  1 here is "mount/dentry pair is in somebody
> > else's namespace or outside of the subtree we are chrooted
> > into".  IOW, what's calling d_path() on your setup?
> 
> No special loads, merely collecting the results after kernel boots up.
> 
> To narrow down, I tested your branch [1] *without* my '%pD' series:
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/log/?h=work.d_path
> 
> The result is the same after kernel boots up.

IOW, you get 1 in call from d_absolute_path().  And the same commit has
-       if (prepend_path(path, &root, &b) > 1)
+       if (unlikely(prepend_path(path, &root, &b) > 1))
there.  What's the problem?

If you want to check mispredictions, put printks at the statements
under those if (unlikely(...)) and see how often do they trigger...