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

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Message-ID: <AM6PR08MB43762B63D11A43FE84849748F7069@AM6PR08MB4376.eurprd08.prod.outlook.com>
Date:   Fri, 25 Jun 2021 08:00:49 +0000
From:   Justin He <Justin.He@....com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     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

Hi Al

> -----Original Message-----
> From: Al Viro <viro@....linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Justin He <Justin.He@....com>; 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: [PATCH 13/14] d_path: prepend_path() is unlikely to return non-
> zero
>
> Signed-off-by: Al Viro <viro@...iv.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 = *p;
        read_seqbegin_or_lock(&rename_lock, &seq);
        error = __prepend_path(path->dentry, real_mount(path->mnt), root, &b);
+       printk("prepend=%d",error);
        if (!(seq & 1))
                rcu_read_unlock();
        if (need_seqretry(&rename_lock, seq)) {

Then the result seems a little different:
root@...os-ampere-02:~# dmesg |grep prepend=1 |wc -l
7417
root@...os-ampere-02:~# dmesg |grep prepend=0 |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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