[PATCH] memcg: lock-free clear page writeback (Was Re: [PATCH04/10] memcg: disable local interrupts in lock_page_cgroup()

[KAMEZAWA Hiroyuki]

Greg, I think clear_page_writeback() will not require _any_ locks with this patch.
But set_page_writeback() requires it...
(Maybe adding a special function for clear_page_writeback() is better rather than
 adding some complex to switch() in update_page_stat())

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Now, at page information accounting, we do lock_page_cgroup() if pc->mem_cgroup
points to a cgroup where someone is moving charges from.

At supporing dirty-page accounting, one of troubles is writeback bit.
In general, writeback can be cleared via IRQ context. To update writeback bit
with lock_page_cgroup() in safe way, we'll have to disable IRQ.
....or do something.

This patch waits for completion of writeback under lock_page() and do
lock_page_cgroup() in safe way. (We never got end_io via IRQ context.)

By this, writeback-accounting will never see race with account_move() and
it can trust pc->mem_cgroup always _without_ any lock.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/memcontrol.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

Index: mmotm-0928/mm/memcontrol.c
===================================================================
--- mmotm-0928.orig/mm/memcontrol.c
+++ mmotm-0928/mm/memcontrol.c
@@ -2183,17 +2183,35 @@ static void __mem_cgroup_move_account(st
 /*
  * check whether the @pc is valid for moving account and call
  * __mem_cgroup_move_account()
+ * Don't call this under pte_lock etc...we'll do lock_page() and wait for
+ * the end of I/O.
  */
 static int mem_cgroup_move_account(struct page_cgroup *pc,
 		struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
 {
 	int ret = -EINVAL;
+
+	/*
+ 	 * We move severl flags and accounting information here. So we need to
+ 	 * avoid the races with update_stat routines. For most of routines,
+ 	 * lock_page_cgroup() is enough for avoiding race. But we need to take
+ 	 * care of IRQ context. If flag updates comes from IRQ context, This
+ 	 * "move account" will be racy (and cause deadlock in lock_page_cgroup())
+ 	 *
+ 	 * Now, the only race we have is Writeback flag. We wait for it cleared
+ 	 * before starting our jobs.
+ 	 */
+
+	lock_page(pc->page);
+	wait_on_page_writeback(pc->page);
+
 	lock_page_cgroup(pc);
 	if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
 		__mem_cgroup_move_account(pc, from, to, uncharge);
 		ret = 0;
 	}
 	unlock_page_cgroup(pc);
+	unlock_page(pc->page);
 	/*
 	 * check events
 	 */

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