Re: [PATCH v3] proc: report open files as size in stat() for /proc/pid/fd

[Brian Foster]

On Mon, Oct 17, 2022 at 09:58:44PM -0700, Ivan Babrou 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/status.
> Unfortunately, counting the number of fds involves iterating the open_files
> bitmap, which has a linear complexity in proportion with the number
> of open files (bitmap slots really, but it's close). We don't want
> to make /proc/pid/status any slower, so instead we 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 with a fallback if it's zero.
> 
> RFC for this patch included iterating open fds under RCU. Thanks
> to Frank Hofmann for the suggestion to use the bitmap instead.
> 
> 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
> ```
> 
> Correctness check:
> 
> ```
> $ sudo ls /proc/1/fd | wc -l
> 65
> ```
> 
> I added the docs for /proc/<pid>/fd while I'm at it.
> 
> Signed-off-by: Ivan Babrou <ivan@xxxxxxxxxxxxxx>
> 
> ---
> v3: Made use of bitmap_weight() to count the bits.
> v2: Added missing rcu_read_lock() / rcu_read_unlock(),
>     task_lock() / task_unlock() and put_task_struct().
> ---
>  Documentation/filesystems/proc.rst | 17 +++++++++++++
>  fs/proc/fd.c                       | 41 ++++++++++++++++++++++++++++++
>  2 files changed, 58 insertions(+)
> 
...
> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> index 913bef0d2a36..439a62c59381 100644
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -279,6 +279,31 @@ 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);
> +	struct fdtable *fdt;
> +	unsigned int open_fds = 0;
> +
> +	if (!p)
> +		return -ENOENT;

Maybe this shouldn't happen, but do you mean to assign the error code to
stat->size in the caller? Otherwise this seems reasonable to me.

Brian

> +
> +	task_lock(p);
> +	if (p->files) {
> +		rcu_read_lock();
> +
> +		fdt = files_fdtable(p->files);
> +		open_fds = bitmap_weight(fdt->open_fds, fdt->max_fds);
> +
> +		rcu_read_unlock();
> +	}
> +	task_unlock(p);
> +
> +	put_task_struct(p);
> +
> +	return open_fds;
> +}
> +
>  static int proc_readfd(struct file *file, struct dir_context *ctx)
>  {
>  	return proc_readfd_common(file, ctx, proc_fd_instantiate);
> @@ -319,9 +344,25 @@ int proc_fd_permission(struct user_namespace *mnt_userns,
>  	return rv;
>  }
>  
> +static 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);
> +
> +	generic_fillattr(&init_user_ns, inode, stat);
> +
> +	/* If it's a directory, put the number of open fds there */
> +	if (S_ISDIR(inode->i_mode))
> +		stat->size = proc_readfd_count(inode);
> +
> +	return 0;
> +}
> +
>  const struct inode_operations proc_fd_inode_operations = {
>  	.lookup		= proc_lookupfd,
>  	.permission	= proc_fd_permission,
> +	.getattr	= proc_fd_getattr,
>  	.setattr	= proc_setattr,
>  };
>  
> -- 
> 2.37.3
>