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

[Justin He]

Hi Al

> -----Original Message-----
> From: Al Viro <viro@xxxxxxxxxxxxxxxx> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Justin He <Justin.He@xxxxxxx>; Petr Mladek <pmladek@xxxxxxxx>; Steven
> Rostedt <rostedt@xxxxxxxxxxx>; Sergey Senozhatsky
> <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx>; Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; Heiko
> Carstens <hca@xxxxxxxxxxxxx>; Vasily Gorbik <gor@xxxxxxxxxxxxx>; Christian
> Borntraeger <borntraeger@xxxxxxxxxx>; Eric W . Biederman
> <ebiederm@xxxxxxxxxxxx>; Darrick J. Wong <darrick.wong@xxxxxxxxxx>; Peter
> Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>; Ira Weiny <ira.weiny@xxxxxxxxx>;
> Eric Biggers <ebiggers@xxxxxxxxxx>; Ahmed S. Darwish
> <a.darwish@xxxxxxxxxxxxx>; open list:DOCUMENTATION <linux-
> doc@xxxxxxxxxxxxxxx>; Linux Kernel Mailing List <linux-
> kernel@xxxxxxxxxxxxxxx>; linux-s390 <linux-s390@xxxxxxxxxxxxxxx>; linux-
> fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>
> Subject: [PATCH 13/14] d_path: prepend_path() is unlikely to return non-
> zero
>
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
>  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@entos-ampere-02:~# dmesg |grep prepend=1 |wc -l
7417
root@entos-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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