On Mon 08-08-11 20:47:38, Johannes Weiner wrote:
[...]

quoted

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2071,7 +2071,6 @@ struct memcg_stock_pcp {
 #define FLUSHING_CACHED_CHARGE	(0)
 };
 static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
-static DEFINE_MUTEX(percpu_charge_mutex);
 
 /*
  * Try to consume stocked charge on this cpu. If success, one page is consumed

@@ -2178,7 +2177,8 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
 
 	for_each_online_cpu(cpu) {
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
-		if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+		if (mem_cgroup_same_or_subtree(root_mem, stock->cached) &&
+				test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
 			flush_work(&stock->work);
 	}
 out:

This hunk triggers a crash for me, as the draining is already done and
stock->cached reset to NULL when dereferenced here.  Oops is attached.

Thanks for catching this. We are racing synchronous drain from
force_empty and async drain from reclaim, I guess. Sync. checked
whether it should wait for the work and the cache got drained and
set to NULL. 
First of all we must not dereference the cached mem without
FLUSHING_CACHED_CHARGE bit test. We have to be sure that there is some
draining on that cache. stock->cached is set to NULL before we clear the
bit (I guess we need to add a barrier into drain_local_stock). So we should
see mem either as NULL or still valid (I have to think some more about
"still valid" part - maybe we will need rcu_read_lock).

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f4ec4e7..626c916 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2197,8 +2197,10 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
 
 	for_each_online_cpu(cpu) {
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
-		if (mem_cgroup_same_or_subtree(root_mem, stock->cached) &&
-				test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+		struct mem_cgroup *mem = stock->cached;
+		if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags) &&
+				 mem && mem_cgroup_same_or_subtree(root_mem, mem)
+				)
 			flush_work(&stock->work);
 	}
 out:

We have this loop in drain_all_stock():

	for_each_online_cpu(cpu) {
		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
		struct mem_cgroup *mem;

		mem = stock->cached;
		if (!mem || !stock->nr_pages)
			continue;
		if (!mem_cgroup_same_or_subtree(root_mem, mem))
			continue;
		if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
			if (cpu == curcpu)
				drain_local_stock(&stock->work);
			else
				schedule_work_on(cpu, &stock->work);
		}
	}

The only thing that stabilizes stock->cached is the knowledge that
there are still pages accounted to the memcg.

Yes you are right we have to set FLUSHING_CACHED_CHARGE before nr_pages
check (and do the appropriate cleanup on the continue paths). This looks
quite ugly, though.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f4ec4e7..eca46141 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2179,17 +2179,23 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
 		struct mem_cgroup *mem;
 
+		/* Try to lock the cache */
+		if(test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+			continue;
+
 		mem = stock->cached;
 		if (!mem || !stock->nr_pages)
-			continue;
+			goto unlock_cache;
 		if (!mem_cgroup_same_or_subtree(root_mem, mem))
-			continue;
-		if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
-			if (cpu == curcpu)
-				drain_local_stock(&stock->work);
-			else
-				schedule_work_on(cpu, &stock->work);
-		}
+			goto unlock_cache;
+
+		if (cpu == curcpu)
+			drain_local_stock(&stock->work);
+		else
+			schedule_work_on(cpu, &stock->work);
+		continue;
+unlock_cache:
+		clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);

                ^^^^^

		need a barrier?
 	}
 
 	if (!sync)
 

Without the mutex serializing this code, can't there be a concurrent
execution that leads to stock->cached being drained, becoming empty
and freed by someone else between the stock->nr_pages check and the
ancestor check, resulting in use after free?

What makes stock->cached safe to dereference?

We are using FLUSHING_CACHED_CHARGE as a lock for local draining. I
guess it should be sufficient.

mutex which was used previously caused that async draining was exclusive
so a root_mem that has potentially many relevant caches has to back off
because other mem wants to clear the cache on the same CPU.

I will think about this tomorrow (with fresh eyes). I think we should be
able to be without mutex.

Anyway thanks for the really good report!

[ 2313.442944] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[ 2313.443935] IP: [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
[ 2313.443935] PGD 4ae7a067 PUD 4adc4067 PMD 0
[ 2313.443935] Oops: 0000 [#1] PREEMPT SMP
[ 2313.443935] CPU 0
[ 2313.443935] Pid: 19677, comm: rmdir Tainted: G        W   3.0.0-mm1-00188-gf38d32b #35 ECS MCP61M-M3/MCP61M-M3
[ 2313.443935] RIP: 0010:[<ffffffff81083b70>]  [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
[ 2313.443935] RSP: 0018:ffff880077b09c88  EFLAGS: 00010202
[ 2313.443935] RAX: ffff8800781bb310 RBX: 0000000000000000 RCX: 000000000000003e
[ 2313.443935] RDX: 0000000000000000 RSI: ffff8800779f7c00 RDI: 0000000000000000
[ 2313.443935] RBP: ffff880077b09c98 R08: ffffffff818a4e88 R09: 0000000000000000
[ 2313.443935] R10: 0000000000000000 R11: dead000000100100 R12: ffff8800779f7c00
[ 2313.443935] R13: ffff8800779f7c00 R14: 0000000000000000 R15: ffff88007bc0eb80
[ 2313.443935] FS:  00007f5d689ec720(0000) GS:ffff88007bc00000(0000) knlGS:0000000000000000
[ 2313.443935] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 2313.443935] CR2: 0000000000000018 CR3: 000000004ad57000 CR4: 00000000000006f0
[ 2313.443935] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2313.443935] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 2313.443935] Process rmdir (pid: 19677, threadinfo ffff880077b08000, task ffff8800781bb310)
[ 2313.443935] Stack:
[ 2313.443935]  ffffffff818a4e88 000000000000eb80 ffff880077b09ca8 ffffffff810feba3
[ 2313.443935]  ffff880077b09d08 ffffffff810feccf ffff880077b09cf8 0000000000000001
[ 2313.443935]  ffff88007bd0eb80 0000000000000001 ffff880077af2000 0000000000000000
[ 2313.443935] Call Trace:
[ 2313.443935]  [<ffffffff810feba3>] mem_cgroup_same_or_subtree+0x33/0x40
[ 2313.443935]  [<ffffffff810feccf>] drain_all_stock+0x11f/0x170
[ 2313.443935]  [<ffffffff81103211>] mem_cgroup_force_empty+0x231/0x6d0
[ 2313.443935]  [<ffffffff81111872>] ? path_put+0x22/0x30
[ 2313.443935]  [<ffffffff8111c925>] ? __d_lookup+0xb5/0x170
[ 2313.443935]  [<ffffffff811036c4>] mem_cgroup_pre_destroy+0x14/0x20
[ 2313.443935]  [<ffffffff81080559>] cgroup_rmdir+0xb9/0x500
[ 2313.443935]  [<ffffffff81063990>] ? abort_exclusive_wait+0xb0/0xb0
[ 2313.443935]  [<ffffffff81114d26>] vfs_rmdir+0x86/0xe0
[ 2313.443935]  [<ffffffff811233d3>] ? mnt_want_write+0x43/0x80
[ 2313.443935]  [<ffffffff81114e7b>] do_rmdir+0xfb/0x110
[ 2313.443935]  [<ffffffff81114ea6>] sys_rmdir+0x16/0x20
[ 2313.443935]  [<ffffffff8154d76b>] system_call_fastpath+0x16/0x1b
[ 2313.443935] Code: b7 42 0a 5d c3 66 0f 1f 44 00 00 55 48 89 e5 48 83 ec 10 48 89 5d f0 4c 89 65 f8 66 66 66 66 90 48 89 fb 49 89 f4 e8 10 85 00 00
[ 2313.443935]  8b 43 18 49 8b 54 24 18 48 85 d2 74 05 48 85 c0 75 15 31 db

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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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