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From:   Justin He <Justin.He@arm.com>
To:     Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Jonathan Corbet <corbet@lwn.net>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Eric Biggers <ebiggers@google.com>,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: RE: [PATCH 13/14] d_path: prepend_path() is unlikely to return
 non-zero
Thread-Topic: [PATCH 13/14] d_path: prepend_path() is unlikely to return
 non-zero
Thread-Index: AQHXTEjX4vA9qb1/tUuDI9sxRSQuQasklodQ
Date:   Fri, 25 Jun 2021 08:00:49 +0000
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Hi Al

> -----Original Message-----
> From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Justin He <Justin.He@arm.com>; Petr Mladek <pmladek@suse.com>; Steven
> Rostedt <rostedt@goodmis.org>; Sergey Senozhatsky
> <senozhatsky@chromium.org>; Andy Shevchenko
> <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
> <linux@rasmusvillemoes.dk>; Jonathan Corbet <corbet@lwn.net>; Heiko
> Carstens <hca@linux.ibm.com>; Vasily Gorbik <gor@linux.ibm.com>; Christia=
n
> Borntraeger <borntraeger@de.ibm.com>; Eric W . Biederman
> <ebiederm@xmission.com>; Darrick J. Wong <darrick.wong@oracle.com>; Peter
> Zijlstra (Intel) <peterz@infradead.org>; Ira Weiny <ira.weiny@intel.com>;
> Eric Biggers <ebiggers@google.com>; Ahmed S. Darwish
> <a.darwish@linutronix.de>; open list:DOCUMENTATION <linux-
> doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
> kernel@vger.kernel.org>; linux-s390 <linux-s390@vger.kernel.org>; linux-
> fsdevel <linux-fsdevel@vger.kernel.org>
> Subject: [PATCH 13/14] d_path: prepend_path() is unlikely to return non-
> zero
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/d_path.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/d_path.c b/fs/d_path.c
> index ba629879a4bf..8a9cd44f6689 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -187,7 +187,7 @@ char *__d_path(const struct path *path,
>       DECLARE_BUFFER(b, buf, buflen);
>
>       prepend(&b, "", 1);
> -     if (prepend_path(path, root, &b) > 0)
> +     if (unlikely(prepend_path(path, root, &b) > 0))
>               return NULL;
>       return extract_string(&b);
>  }
> @@ -199,7 +199,7 @@ char *d_absolute_path(const struct path *path,
>       DECLARE_BUFFER(b, buf, buflen);
>
>       prepend(&b, "", 1);
> -     if (prepend_path(path, &root, &b) > 1)
> +     if (unlikely(prepend_path(path, &root, &b) > 1))
>               return ERR_PTR(-EINVAL);
>       return extract_string(&b);
>  }
> @@ -396,7 +396,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned
> long, size)
>               DECLARE_BUFFER(b, page, PATH_MAX);
>
>               prepend(&b, "", 1);
> -             if (prepend_path(&pwd, &root, &b) > 0)
> +             if (unlikely(prepend_path(&pwd, &root, &b) > 0))
>                       prepend(&b, "(unreachable)", 13);
>               rcu_read_unlock();
>
> --
> 2.11.0
I tested it with a debugging patch as follows:
diff --git a/fs/d_path.c b/fs/d_path.c
index aea254ac9e1f..8eecd04be7bb 100644
--- a/fs/d_path.c
+++ b/fs/d_path.c
@@ -210,6 +210,7 @@ static int prepend_path(const struct path *path,
        b =3D *p;
        read_seqbegin_or_lock(&rename_lock, &seq);
        error =3D __prepend_path(path->dentry, real_mount(path->mnt), root,=
 &b);
+       printk("prepend=3D%d",error);
        if (!(seq & 1))
                rcu_read_unlock();
        if (need_seqretry(&rename_lock, seq)) {

Then the result seems a little different:
root@entos-ampere-02:~# dmesg |grep prepend=3D1 |wc -l
7417
root@entos-ampere-02:~# dmesg |grep prepend=3D0 |wc -l
772

The kernel is 5.13.0-rc2+ + this series + my '%pD' series

Any thoughts?

---
Cheers,
Jia He (Justin)


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