Re: [PATCH 4/4] memcg: get rid of percpu_charge_mutex lock

[Johannes Weiner <jweiner@redhat.com>]

On Fri, Jul 22, 2011 at 01:20:25PM +0200, Michal Hocko wrote:
> percpu_charge_mutex protects from multiple simultaneous per-cpu charge
> caches draining because we might end up having too many work items.
> At least this was the case until 26fe6168 (memcg: fix percpu cached
> charge draining frequency) when we introduced a more targeted draining
> for async mode.
> Now that also sync draining is targeted we can safely remove mutex
> because we will not send more work than the current number of CPUs.
> FLUSHING_CACHED_CHARGE protects from sending the same work multiple
> times and stock->nr_pages == 0 protects from pointless sending a work
> if there is obviously nothing to be done. This is of course racy but we
> can live with it as the race window is really small (we would have to
> see FLUSHING_CACHED_CHARGE cleared while nr_pages would be still
> non-zero).
> The only remaining place where we can race is synchronous mode when we
> rely on FLUSHING_CACHED_CHARGE test which might have been set by other
> drainer on the same group but we should wait in that case as well.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  mm/memcontrol.c |   12 ++----------
>  1 files changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 3685107..f8463a0 100644
> --- 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.

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.

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?

[ 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