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

[Ivan Babrou <ivan@cloudflare.com>]

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.

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
```

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

I can probably figure out how to do the former, but the latter
will require somebody with more experience in file code than myself.

Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
---
 fs/proc/fd.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 913bef0d2a36..c7ac142500a8 100644
--- 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);
+
+	/* 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,
 };
 
-- 
2.37.2