Re: [PATCH] NFS: Add "device" tag to /proc/self/mountstats

[Bryan Schumaker <bjschuma@netapp.com>]

On 08/29/2011 11:36 AM, Chuck Lever wrote:
> 
> On Aug 29, 2011, at 10:55 AM, Bryan Schumaker wrote:
> 
>> On 08/29/2011 10:43 AM, Chuck Lever wrote:
>>>
>>> On Aug 29, 2011, at 9:46 AM, Bryan Schumaker wrote:
>>>
>>>> On 08/27/2011 07:52 PM, Chuck Lever wrote:
>>>>>
>>>>> On Aug 26, 2011, at 4:52 PM, Bryan Schumaker wrote:
>>>>>
>>>>>> nfsiostat was failing to find mounted filesystems on recent kernels due
>>>>>> to changes in the VFS that resulted in a missing "device" tag in the
>>>>>> /proc/self/mountstats file.
>>>>>
>>>>> What was the commit ID that changed this formal kernel API?  I'd like to see the justification.
>>>>
>>>> It was changed by this commit:
>>>>
>>>> commit c7f404b40a3665d9f4e9a927cc5c1ee0479ed8f9
>>>> Author: Al Viro <viro@zeniv.linux.org.uk>
>>>> Date:   Wed Mar 16 06:59:40 2011 -0400
>>>>
>>>>   vfs: new superblock methods to override /proc/*/mount{s,info}
>>>>
>>>>   a) ->show_devname(m, mnt) - what to put into devname columns in mounts,
>>>>   mountinfo and mountstats
>>>>   b) ->show_path(m, mnt) - what to put into relative path column in mountinfo
>>>>
>>>>   Leaving those NULL gives old behaviour.  NFS switched to using those.
>>>>
>>>>   Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
>>>
>>> It looks like dropping the "device" tag from /proc/self/mountstats was not intentional, so this is a kernel API regression.  At a guess, 2.6.39 probably works right, but 3.0 is broken.  Whatever solution we come up with might need to be cc:d to stable@kernel.org.  Be sure to verify which released kernels are broken.
>>
>> I'll double check that 39 works.
>>>
>>> Does changing nfs_show_devname() to add a "device" tag have any effect on the other two /proc files (mounts and mountinfo)?  I suspect you do want to fix show_vfsstat() instead of nfs_show_devname().  This should add "device" only where it is needed, and would work for all file systems.
>>
>> Yeah, looks like it changes both.  I'm looking at show_vfsstat() now, but nfs_show_devname() will return an error if a call to nfs_path() fails.  This would probably result in an empty "device " line printed to /proc/self/mountstats.  Do you know if that would be a problem for anything?
> 
> The only error return from nfs_path() is if the name is too long.  You could change nfs_show_devname() to output a fixed error string in that case (like "path too long"), or something even more generic.

Sounds good to me, I'll work in something.

> 
>> I'm thinking about doing something like this:
>>
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 22bfe82..1429114 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -1109,6 +1109,7 @@ static int show_vfsstat(struct seq_file *m, void *v)
>>
>>        /* device */
>>        if (mnt->mnt_sb->s_op->show_devname) {
>> +               seq_puts(m, "device ");
>>                err = mnt->mnt_sb->s_op->show_devname(m, mnt);
>>        } else {
>>                if (mnt->mnt_devname) {
> 
> There probably isn't a clean way to have just one seq_puts(m, "device "); invocation in this function.

My thoughts exactly.  I'll stick with having two seq_puts(m, "device ") calls since it's easier to figure out what's going on that way.

> 
>>
>>
>>>
>>>>
>>>>
>>>>>
>>>>>> This patch re-adds the "device" tag for NFS
>>>>>> mount data in /proc/self/mountstats.
>>>>>
>>>>> The purpose of mountstats is to be an interface that all file systems can use.  If we can't add "device" back to all mounts listed in /proc/self/mountstats, it might be better to adjust nfsiostats to cope.
>>>>>
>>>>>> Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
>>>>>> ---
>>>>>> fs/nfs/super.c |    4 +++-
>>>>>> 1 files changed, 3 insertions(+), 1 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/nfs/super.c b/fs/nfs/super.c
>>>>>> index b961cea..e6a0317 100644
>>>>>> --- a/fs/nfs/super.c
>>>>>> +++ b/fs/nfs/super.c
>>>>>> @@ -766,8 +766,10 @@ static int nfs_show_devname(struct seq_file *m, struct vfsmount *mnt)
>>>>>> 	devname = nfs_path(&dummy, mnt->mnt_root, page, PAGE_SIZE);
>>>>>> 	if (IS_ERR(devname))
>>>>>> 		err = PTR_ERR(devname);
>>>>>> -	else
>>>>>> +	else {
>>>>>> +		seq_puts(m, "device ");
>>>>>> 		seq_escape(m, devname, " \t\n\\");
>>>>>> +	}
>>>>>> 	free_page((unsigned long)page);
>>>>>> 	return err;
>>>>>> }
>>>>>> -- 
>>>>>> 1.7.6
>>>>>
>>>>
>>>
>>
>