[PATCH 6/7] proc: explain which entries can't be mountpoints

[Christian Brauner <brauner@kernel.org>]

Document how a failing ->d_revalidate() takes the mounts off a procfs
entry, which entries can't be mountpoints because of that and why the
other entries of a process are fine.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/proc/internal.h | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index b9aaac41c283..296ea13bbd49 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -425,9 +425,21 @@ static inline void pde_force_lookup(struct proc_dir_entry *pde)
 }
 
 /*
- * Add a new procfs dentry that can't serve as a mountpoint. That should
- * encompass anything that is ephemeral and can just disappear while the
- * process is still around.
+ * Add a new procfs dentry that can't serve as a mountpoint.
+ *
+ * If ->d_revalidate() fails the VFS calls d_invalidate() on the dentry. That
+ * detaches every mount on it and below it, in every mount namespace, locked
+ * mounts included, and the next lookup creates a new dentry without them. So
+ * nothing whose ->d_revalidate() can fail while the process is still around
+ * may be a mountpoint: the entries below fd/ and fdinfo/ once the fd is
+ * closed, the entries below map_files/ once the mapping is gone and
+ * everything below net/ where proc_net_d_revalidate() fails on every lookup.
+ * The links into /proc/self can't be mountpoints either. A mount on one of
+ * them would redirect every process that walks through it.
+ *
+ * The other entries of a process fail pid_revalidate() only once the task
+ * is gone and proc_flush_pid() invalidates them when it is reaped. Mounts
+ * on them go away together with the task.
  */
 static inline struct dentry *proc_splice_unmountable(struct inode *inode,
 		struct dentry *dentry, const struct dentry_operations *d_ops)

-- 
2.53.0