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

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.

[ivan@cloudflare.com: use bitmap_weight() to count the bits]
  Link: https://lkml.kernel.org/r/20221018045844.37697-1-ivan@cloudflare.com
[akpm@linux-foundation.org: include linux/bitmap.h for bitmap_weight()]
[ivan@cloudflare.com: return errno from proc_fd_getattr() instead of setting negative size]
  Link: https://lkml.kernel.org/r/20221024173140.30673-1-ivan@cloudflare.com
Link: https://lkml.kernel.org/r/20220922224027.59266-1-ivan@cloudflare.com
Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Ivan Babrou <ivan@cloudflare.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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
 =============================
 

fs/proc/fd.c

@@ -7,6 +7,7 @@
 #include <linux/namei.h>
 #include <linux/pid.h>
 #include <linux/ptrace.h>
+#include <linux/bitmap.h>
 #include <linux/security.h>
 #include <linux/file.h>
 #include <linux/seq_file.h>
@@ -279,6 +280,30 @@ out:
 	return 0;
 }
 
+static int proc_readfd_count(struct inode *inode, loff_t *count)
+{
+	struct task_struct *p = get_proc_task(inode);
+	struct fdtable *fdt;
+
+	if (!p)
+		return -ENOENT;
+
+	task_lock(p);
+	if (p->files) {
+		rcu_read_lock();
+
+		fdt = files_fdtable(p->files);
+		*count = bitmap_weight(fdt->open_fds, fdt->max_fds);
+
+		rcu_read_unlock();
+	}
+	task_unlock(p);
+
+	put_task_struct(p);
+
+	return 0;
+}
+
 static int proc_readfd(struct file *file, struct dir_context *ctx)
 {
 	return proc_readfd_common(file, ctx, proc_fd_instantiate);
@@ -319,9 +344,29 @@ 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);
+	int rv = 0;
+
+	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)) {
+		rv = proc_readfd_count(inode, &stat->size);
+		if (rv < 0)
+			return rv;
+	}
+
+	return rv;
+}
+
 const struct inode_operations proc_fd_inode_operations = {
 	.lookup		= proc_lookupfd,
 	.permission	= proc_fd_permission,
+	.getattr	= proc_fd_getattr,
 	.setattr	= proc_setattr,
 };