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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