[PATCH] pidhash: Refactor the pid hash table

Simplifies the code, reduces the need for 4 pid hash tables, and makes the
code more capable.

In the discussions I had with Oleg it was felt that to a large extent the
cleanup itself justified the work.  With struct pid being dynamically
allocated meant we could create the hash table entry when the pid was
allocated and free the hash table entry when the pid was freed.  Instead of
playing with the hash lists when ever a process would attach or detach to a
process.

For myself the fact that it gave what my previous task_ref patch gave for free
with simpler code was a big win.  The problem is that if you hold a reference
to struct task_struct you lock in 10K of low memory.  If you do that in a user
controllable way like /proc does, with an unprivileged but hostile user space
application with typical resource limits of 1000 fds and 100 processes I can
trigger the OOM killer by consuming all of low memory with task structs, on a
machine wight 1GB of low memory.

If I instead hold a reference to struct pid which holds a pointer to my
task_struct, I don't suffer from that problem because struct pid is 2 orders
of magnitude smaller.  In fact struct pid is small enough that most other
kernel data structures dwarf it, so simply limiting the number of referring
data structures is enough to prevent exhaustion of low memory.

This splits the current struct pid into two structures, struct pid and struct
pid_link, and reduces our number of hash tables from PIDTYPE_MAX to just one.
struct pid_link is the per process linkage into the hash tables and lives in
struct task_struct.  struct pid is given an indepedent lifetime, and holds
pointers to each of the pid types.

The independent life of struct pid simplifies attach_pid, and detach_pid,
because we are always manipulating the list of pids and not the hash table.
In addition in giving struct pid an indpendent life it makes the concept much
more powerful.

Kernel data structures can now embed a struct pid * instead of a pid_t and
not suffer from pid wrap around problems or from keeping unnecessarily
large amounts of memory allocated.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 83 insertions, 17 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 5b9082cc600..29960b03bef 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_PID_H
 #define _LINUX_PID_H
 
+#include <linux/rcupdate.h>
+
 enum pid_type
 {
 	PIDTYPE_PID,
@@ -9,45 +11,109 @@ enum pid_type
 	PIDTYPE_MAX
 };
 
+/*
+ * What is struct pid?
+ *
+ * A struct pid is the kernel's internal notion of a process identifier.
+ * It refers to individual tasks, process groups, and sessions.  While
+ * there are processes attached to it the struct pid lives in a hash
+ * table, so it and then the processes that it refers to can be found
+ * quickly from the numeric pid value.  The attached processes may be
+ * quickly accessed by following pointers from struct pid.
+ *
+ * Storing pid_t values in the kernel and refering to them later has a
+ * problem.  The process originally with that pid may have exited and the
+ * pid allocator wrapped, and another process could have come along
+ * and been assigned that pid.
+ *
+ * Referring to user space processes by holding a reference to struct
+ * task_struct has a problem.  When the user space process exits
+ * the now useless task_struct is still kept.  A task_struct plus a
+ * stack consumes around 10K of low kernel memory.  More precisely
+ * this is THREAD_SIZE + sizeof(struct task_struct).  By comparison
+ * a struct pid is about 64 bytes.
+ *
+ * Holding a reference to struct pid solves both of these problems.
+ * It is small so holding a reference does not consume a lot of
+ * resources, and since a new struct pid is allocated when the numeric
+ * pid value is reused we don't mistakenly refer to new processes.
+ */
+
 struct pid
 {
+	atomic_t count;
 	/* Try to keep pid_chain in the same cacheline as nr for find_pid */
 	int nr;
 	struct hlist_node pid_chain;
-	/* list of pids with the same nr, only one of them is in the hash */
-	struct list_head pid_list;
+	/* lists of tasks that use this pid */
+	struct hlist_head tasks[PIDTYPE_MAX];
+	struct rcu_head rcu;
 };
 
-#define pid_task(elem, type) \
-	list_entry(elem, struct task_struct, pids[type].pid_list)
+struct pid_link
+{
+	struct hlist_node node;
+	struct pid *pid;
+};
+
+static inline struct pid *get_pid(struct pid *pid)
+{
+	if (pid)
+		atomic_inc(&pid->count);
+	return pid;
+}
+
+extern void FASTCALL(put_pid(struct pid *pid));
+extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
+extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
+						enum pid_type));
 
 /*
  * attach_pid() and detach_pid() must be called with the tasklist_lock
  * write-held.
  */
-extern int FASTCALL(attach_pid(struct task_struct *task, enum pid_type type, int nr));
+extern int FASTCALL(attach_pid(struct task_struct *task,
+				enum pid_type type, int nr));
 
 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
 
 /*
  * look up a PID in the hash table. Must be called with the tasklist_lock
- * held.
+ * or rcu_read_lock() held.
+ */
+extern struct pid *FASTCALL(find_pid(int nr));
+
+/*
+ * Lookup a PID in the hash table, and return with it's count elevated.
  */
-extern struct pid *FASTCALL(find_pid(enum pid_type, int));
+extern struct pid *find_get_pid(int nr);
 
-extern int alloc_pidmap(void);
-extern void FASTCALL(free_pidmap(int));
+extern struct pid *alloc_pid(void);
+extern void FASTCALL(free_pid(struct pid *pid));
 
+#define pid_next(task, type)					\
+	((task)->pids[(type)].node.next)
+
+#define pid_next_task(task, type) 				\
+	hlist_entry(pid_next(task, type), struct task_struct,	\
+			pids[(type)].node)
+
+
+/* We could use hlist_for_each_entry_rcu here but it takes more arguments
+ * than the do_each_task_pid/while_each_task_pid.  So we roll our own
+ * to preserve the existing interface.
+ */
 #define do_each_task_pid(who, type, task)				\
 	if ((task = find_task_by_pid_type(type, who))) {		\
-		prefetch((task)->pids[type].pid_list.next);		\
+		prefetch(pid_next(task, type));				\
 		do {
 
 #define while_each_task_pid(who, type, task)				\
-		} while (task = pid_task((task)->pids[type].pid_list.next,\
-						type),			\
-			prefetch((task)->pids[type].pid_list.next),	\
-			hlist_unhashed(&(task)->pids[type].pid_chain));	\
-	}								\
+		} while (pid_next(task, type) &&  ({			\
+				task = pid_next_task(task, type);	\
+				rcu_dereference(task);			\
+				prefetch(pid_next(task, type));		\
+				1; }) );				\
+	}
 
 #endif /* _LINUX_PID_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 7e0ff5dba98..541f4828f5e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -760,7 +760,7 @@ struct task_struct {
 	struct task_struct *group_leader;	/* threadgroup leader */
 
 	/* PID/PID hash table linkage. */
-	struct pid pids[PIDTYPE_MAX];
+	struct pid_link pids[PIDTYPE_MAX];
 	struct list_head thread_group;
 
 	struct completion *vfork_done;		/* for vfork() */
@@ -899,7 +899,7 @@ static inline pid_t process_group(struct task_struct *tsk)
  */
 static inline int pid_alive(struct task_struct *p)
 {
-	return p->pids[PIDTYPE_PID].nr != 0;
+	return p->pids[PIDTYPE_PID].pid != NULL;
 }
 
 extern void free_task(struct task_struct *tsk);