Re: [PATCH] perf trace beautify: Beautify flags of mount(2) and umount(2).

[Benjamin Peterson <benjamin@python.org>]

Thanks for the review.

On Thu, Aug 30, 2018, at 11:28, Arnaldo Carvalho de Melo wrote:
> Em Mon, Aug 27, 2018 at 08:53:44PM -0700, Benjamin Peterson escreveu:
> > Example output of perf trace -e mount,umount2:
> > 
> >   6243.930 ( 0.052 ms): mount/30976 mount(dev_name: 0x55dc541bb920, dir_name: 0x55dc541bc960, type: 0x55dc541b9c40, flags: BIND) = 0
> >   7851.821 (26.015 ms): umount/30983 umount2(name: 0x558daa82cf50, flags: MNT_FORCE                        ) = 0
> > 
> > Signed-off-by: Benjamin Peterson <benjamin@python.org>
> 
> So, I tried:
> 
> Before your patch:
> 
>   # perf trace -e *mount* mount /dev/mapper/fedora-home /s
>      0.000 ( 4.335 ms): mount/3126 mount(dev_name: 0x55deecc4d480, 
> dir_name: 0x55deecc4d4a0, type: 0x55deecc53880, flags: 3236757504) = 0
>   #
>   # perf trace -e *mount* umount /dev/mapper/fedora-home /s
>          ? (         ): umount/3138  ... [continued]: umount2()) = 0
>     11.576 (umount:  0.004 ms): /s: not mounted.umount/
> 3138 umount2(arg0: 94501956754656, arg1: 0, arg2: 1, arg3: 
> 140051050083104, arg4: 4, arg5: 94501956755136) = -1 EINVAL Invalid 
> argument
>   #
> 
> After:
> 
> [root@jouet ~]# perf trace -e *mount* mount /dev/mapper/fedora-home /s
>      0.000 ( 1.213 ms): mount/5244 mount(dev_name: 0x5558c1169480, 
> dir_name: 0x5558c11694a0, type: 0x5558c116f880, flags: ) = 0
> [root@jouet ~]# perf trace -e *mount* umount /s
>      0.000 ( 9.241 ms): umount/5251 umount2(name: 
> 0x55f74a986480                                          ) = 0
> 
> The flags for mount got empty? Can you pleaes check that? I.e. using the
> default mount options, not specifying a 'bind' mount like you did.

That comes from these lines in syscall_arg__scnprintf_mount_flags:

if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
	flags &= ~MS_MGC_MSK;

The reason for this is explained in the comment for do_mount in fs/namespace.c:

/*
 * Pre-0.97 versions of mount() didn't have a flags word.
 * When the flags word was introduced its top half was required
 * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9.
 * Therefore, if this magic number is present, it carries no information
 * and must be discarded.
*/

(I used a bind mount for my commit message example because a "default" mount() doesn't use any flags.)

> 
> I'm splitting this patch so that the aliasing of 'umount2' to 'umount'
> gets merged now,
> 
> Check my perf/core branch at:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git

LGTM. Thanks