[PATCH v3] 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/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@cloudflare.com>

---
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/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 898c99eae8e4..ec6cfdf1796a 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -47,6 +47,7 @@ fixes/update part 1.1  Stefani Seibold <stefani@seibold.net>    June 9 2009
   3.10  /proc/<pid>/timerslack_ns - Task timerslack value
   3.11	/proc/<pid>/patch_state - Livepatch patch operation state
   3.12	/proc/<pid>/arch_status - Task architecture specific information
+  3.13  /proc/<pid>/fd - List of symlinks to open files
 
   4	Configuring procfs
   4.1	Mount options
@@ -2149,6 +2150,22 @@ AVX512_elapsed_ms
   the task is unlikely an AVX512 user, but depends on the workload and the
   scheduling scenario, it also could be a false negative mentioned above.
 
+3.13 /proc/<pid>/fd - List of symlinks to open files
+-------------------------------------------------------
+This directory contains symbolic links which represent open files
+the process is maintaining.  Example output::
+
+  lr-x------ 1 root root 64 Sep 20 17:53 0 -> /dev/null
+  l-wx------ 1 root root 64 Sep 20 17:53 1 -> /dev/null
+  lrwx------ 1 root root 64 Sep 20 17:53 10 -> 'socket:[12539]'
+  lrwx------ 1 root root 64 Sep 20 17:53 11 -> 'socket:[12540]'
+  lrwx------ 1 root root 64 Sep 20 17:53 12 -> 'socket:[12542]'
+
+The number of open files for the process is stored in 'size' member
+of stat() output for /proc/<pid>/fd for fast access.
+-------------------------------------------------------
+
+
 Chapter 4: Configuring procfs
 =============================
 
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;
+
+	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