Hi Al

-----Original Message-----
From: Al Viro <redacted> On Behalf Of Al Viro
Sent: Monday, June 28, 2021 12:14 PM
To: Justin He <redacted>
Cc: Linus Torvalds <torvalds@linux-foundation.org>; Petr Mladek
[off-list ref]; Steven Rostedt [off-list ref]; Sergey
Senozhatsky [off-list ref]; Andy Shevchenko
[off-list ref]; Rasmus Villemoes
[off-list ref]; Jonathan Corbet [off-list ref]; Heiko
Carstens [off-list ref]; Vasily Gorbik [off-list ref]; Christian
Borntraeger [off-list ref]; Eric W . Biederman
[off-list ref]; Darrick J. Wong [off-list ref]; Peter
Zijlstra (Intel) [off-list ref]; Ira Weiny [off-list ref];
Eric Biggers [off-list ref]; Ahmed S. Darwish
[off-list ref]; open list:DOCUMENTATION <linux-
doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
kernel@vger.kernel.org>; linux-s390 [off-list ref]; linux-
fsdevel [off-list ref]
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:

quoted

quoted

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

quoted

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?

Ah, sorry for the mistake.


--
Cheers,
Justin (Jia He)

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

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