Re: [RFC] proc: report open files as size in stat() for /proc/pid/fd

[Christian Brauner]

On Sat, Sep 17, 2022 at 10:15:13AM +0300, Alexey Dobriyan wrote:
> On Fri, Sep 16, 2022 at 05:01:15PM -0700, Andrew Morton wrote:
> > (cc's added)
> > 
> > On Fri, 16 Sep 2022 16:08:52 -0700 Ivan Babrou <ivan@xxxxxxxxxxxxxx> wrote:
> > 
> > > Many monitoring tools include open file count as a metric. Currently
> > > the only way to get this number is to enumerate the files in /proc/pid/fd.
> > > 
> > > The problem with the current approach is that it does many things people
> > > generally don't care about when they need one number for a metric.
> > > In our tests for cadvisor, which reports open file counts per cgroup,
> > > we observed that reading the number of open files is slow. Out of 35.23%
> > > of CPU time spent in `proc_readfd_common`, we see 29.43% spent in
> > > `proc_fill_cache`, which is responsible for filling dentry info.
> > > Some of this extra time is spinlock contention, but it's a contention
> > > for the lock we don't want to take to begin with.
> > > 
> > > We considered putting the number of open files in /proc/pid/stat.
> > > Unfortunately, counting the number of fds involves iterating the fdtable,
> > > which means that it might slow down /proc/pid/stat for processes
> > > with many open files. Instead we opted to put this info in /proc/pid/fd
> > > as a size member of the stat syscall result. Previously the reported
> > > number was zero, so there's very little risk of breaking anything,
> > > while still providing a somewhat logical way to count the open files.
> > 
> > Documentation/filesystems/proc.rst would be an appropriate place to
> > document this ;)
> > 
> > > Previously:
> > > 
> > > ```
> > > $ sudo stat /proc/1/fd | head -n2
> > >   File: /proc/1/fd
> > >   Size: 0         	Blocks: 0          IO Block: 1024   directory
> > > ```
> > > 
> > > With this patch:
> > > 
> > > ```
> > > $ sudo stat /proc/1/fd | head -n2
> > >   File: /proc/1/fd
> > >   Size: 65        	Blocks: 0          IO Block: 1024   directory
> 
> Yes. This is natural place.
> 
> > > ```
> > > 
> > > Correctness check:
> > > 
> > > ```
> > > $ sudo ls /proc/1/fd | wc -l
> > > 65
> > > ```
> > > 
> > > There are two alternatives to this approach that I can see:
> > > 
> > > * Expose /proc/pid/fd_count with a count there
> 
> > > * Make fd count acces O(1) and expose it in /proc/pid/status
> 
> This is doable, next to FDSize.
> 
> Below is doable too.
> 
> > > --- a/fs/proc/fd.c
> > > +++ b/fs/proc/fd.c
> > > @@ -279,6 +279,29 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
> > >  	return 0;
> > >  }
> > >  
> > > +static int proc_readfd_count(struct inode *inode)
> > > +{
> > > +	struct task_struct *p = get_proc_task(inode);
> > > +	unsigned int fd = 0, count = 0;
> > > +
> > > +	if (!p)
> > > +		return -ENOENT;
> > > +
> > > +	rcu_read_lock();
> > > +	while (task_lookup_next_fd_rcu(p, &fd)) {
> > > +		rcu_read_unlock();
> > > +
> > > +		count++;
> > > +		fd++;
> > > +
> > > +		cond_resched();
> > > +		rcu_read_lock();
> > > +	}
> > > +	rcu_read_unlock();
> > > +	put_task_struct(p);
> > > +	return count;
> > > +}
> > > +
> > >  static int proc_readfd(struct file *file, struct dir_context *ctx)
> > >  {
> > >  	return proc_readfd_common(file, ctx, proc_fd_instantiate);
> > > @@ -319,9 +342,33 @@ int proc_fd_permission(struct user_namespace *mnt_userns,
> > >  	return rv;
> > >  }
> > >  
> > > +int proc_fd_getattr(struct user_namespace *mnt_userns,
> > > +			const struct path *path, struct kstat *stat,
> > > +			u32 request_mask, unsigned int query_flags)
> > > +{
> > > +	struct inode *inode = d_inode(path->dentry);
> > > +	struct proc_dir_entry *de = PDE(inode);
> > > +
> > > +	if (de) {
> > > +		nlink_t nlink = READ_ONCE(de->nlink);
> > > +
> > > +		if (nlink > 0)
> > > +			set_nlink(inode, nlink);
> > > +	}
> > > +
> > > +	generic_fillattr(&init_user_ns, inode, stat);
> 			 ^^^^^^^^^^^^^
> 
> Is this correct? I'm not userns guy at all.

This is correct. :)