[PATCH] (3/5) sane procfs/dcache interaction

 - sane dentry retention.  Namely, we don't kill /proc/<pid> dentries at the
   first opportunity (as the current tree does).  Instead we do the following:
	* ->d_delete() kills it only if process is already dead.
	* all ->lookup() in proc/base.c end with checking if process is still
	  alive and unhash if it isn't.
	* proc_pid_lookup() (lookup for /proc/<pid>) caches reference to dentry
	  in task_struct.  It's _not_ counted in ->d_count.
	* ->d_iput() resets said reference to NULL.
	* release_task() (burying a zombie) checks if there is a cached
	  reference and if there is - shrinks the subtree.
	* tasklist_lock is used for exclusion.
   That way we are guaranteed that after release_task() all dentries in
   /proc/<pid> will go away as soon as possible; OTOH, before release_task()
   we have normal retention policy - they go away under memory pressure with
   the same rules as for dentries on any other fs.

2 files changed, 16 insertions, 0 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 58b05e3ef9f8..e3fdd078f449 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -29,13 +29,28 @@ int getrusage(struct task_struct *, int, struct rusage *);
 
 static inline void __unhash_process(struct task_struct *p)
 {
+	struct dentry *proc_dentry;
 	write_lock_irq(&tasklist_lock);
 	nr_threads--;
 	unhash_pid(p);
 	REMOVE_LINKS(p);
 	list_del(&p->thread_group);
 	p->pid = 0;
+	proc_dentry = p->proc_dentry;
+	if (unlikely(proc_dentry)) {
+		spin_lock(&dcache_lock);
+		if (!list_empty(&proc_dentry->d_hash)) {
+			dget_locked(proc_dentry);
+			list_del_init(&proc_dentry->d_hash);
+		} else
+			proc_dentry = NULL;
+		spin_unlock(&dcache_lock);
+	}
 	write_unlock_irq(&tasklist_lock);
+	if (unlikely(proc_dentry)) {
+		shrink_dcache_parent(proc_dentry);
+		dput(proc_dentry);
+	}
 }
 
 static void release_task(struct task_struct * p)
diff --git a/kernel/fork.c b/kernel/fork.c
index d7a274cbec95..380c1bafe75c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -665,6 +665,7 @@ int do_fork(unsigned long clone_flags, unsigned long stack_start,
 
 	copy_flags(clone_flags, p);
 	p->pid = get_pid(clone_flags);
+	p->proc_dentry = NULL;
 
 	INIT_LIST_HEAD(&p->run_list);