linux-kernel - Re: [PATCH] mm: workingset: fix NULL ptr in count_shadow

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Date:   Thu, 1 Dec 2016 16:32:41 +0300
From:   Vladimir Davydov <vdavydov@...antool.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Marek Marczykowski-Górecki 
        <marmarek@...uw.edu.pl>, Balbir Singh <bsingharora@...il.com>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: workingset: fix NULL ptr in count_shadow_nodes

On Thu, Dec 01, 2016 at 02:21:56PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> 0a6b76dd23fa ("mm: workingset: make shadow node shrinker memcg aware")
> has made the workingset shadow nodes shrinker memcg aware. The
> implementation is not correct though because memcg_kmem_enabled() might
> become true while we are doing a global reclaim when the sc->memcg might
> be NULL which is exactly what Marek has seen:
> 
> [   15.665196] BUG: unable to handle kernel NULL pointer dereference at 0000000000000400
> [   15.665213] IP: [<ffffffff8122d520>] mem_cgroup_node_nr_lru_pages+0x20/0x40
> [   15.665225] PGD 0
> [   15.665230] Oops: 0000 [#1] SMP
> [   15.665285] CPU: 0 PID: 60 Comm: kswapd0 Tainted: G           O   4.8.10-12.pvops.qubes.x86_64 #1
> [   15.665292] task: ffff880011863b00 task.stack: ffff880011868000
> [   15.665297] RIP: e030:[<ffffffff8122d520>]  [<ffffffff8122d520>] mem_cgroup_node_nr_lru_pages+0x20/0x40
> [   15.665307] RSP: e02b:ffff88001186bc70  EFLAGS: 00010293
> [   15.665311] RAX: 0000000000000000 RBX: ffff88001186bd20 RCX: 0000000000000002
> [   15.665317] RDX: 000000000000000c RSI: 0000000000000000 RDI: 0000000000000000
> [   15.665322] RBP: ffff88001186bc70 R08: 28f5c28f5c28f5c3 R09: 0000000000000000
> [   15.665327] R10: 0000000000006c34 R11: 0000000000000333 R12: 00000000000001f6
> [   15.665332] R13: ffffffff81c6f6a0 R14: 0000000000000000 R15: 0000000000000000
> [   15.665343] FS:  0000000000000000(0000) GS:ffff880013c00000(0000) knlGS:ffff880013d00000
> [   15.665351] CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   15.665358] CR2: 0000000000000400 CR3: 00000000122f2000 CR4: 0000000000042660
> [   15.665366] Stack:
> [   15.665371]  ffff88001186bc98 ffffffff811e0dda 00000000000002eb 0000000000000080
> [   15.665384]  ffffffff81c6f6a0 ffff88001186bd70 ffffffff811c36d9 0000000000000000
> [   15.665397]  ffff88001186bcb0 ffff88001186bcb0 ffff88001186bcc0 000000000000abc5
> [   15.665410] Call Trace:
> [   15.665419]  [<ffffffff811e0dda>] count_shadow_nodes+0x9a/0xa0
> [   15.665428]  [<ffffffff811c36d9>] shrink_slab.part.42+0x119/0x3e0
> [   15.666049]  [<ffffffff811c83ec>] shrink_node+0x22c/0x320
> [   15.666049]  [<ffffffff811c928c>] kswapd+0x32c/0x700
> [   15.666049]  [<ffffffff811c8f60>] ? mem_cgroup_shrink_node+0x180/0x180
> [   15.666049]  [<ffffffff810c1b08>] kthread+0xd8/0xf0
> [   15.666049]  [<ffffffff817a3abf>] ret_from_fork+0x1f/0x40
> [   15.666049]  [<ffffffff810c1a30>] ? kthread_create_on_node+0x190/0x190
> [   15.666049] Code: 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 3b 35 dd eb b1 00 55 48 89 e5 73 2c 89 d2 31 c9 31 c0 4c 63 ce 48 0f a3 ca 73 13 <4a> 8b b4 cf 00 04 00 00 41 89 c8 4a 03 84 c6 80 00 00 00 83 c1
> [   15.666049] RIP  [<ffffffff8122d520>] mem_cgroup_node_nr_lru_pages+0x20/0x40
> [   15.666049]  RSP <ffff88001186bc70>
> [   15.666049] CR2: 0000000000000400
> [   15.666049] ---[ end trace 100494b9edbdfc4d ]---
> 
> This patch fixes the issue by checking sc->memcg rather than memcg_kmem_enabled()
> which is sufficient because shrink_slab makes sure that only memcg aware shrinkers
> will get non-NULL memcgs and only if memcg_kmem_enabled is true.
> 
> Fixes: 0a6b76dd23fa ("mm: workingset: make shadow node shrinker memcg aware")
> Reported-and-tested-by: Marek Marczykowski-Górecki <marmarek@...uw.edu.pl>
> Cc: stable@...r.kernel.org # 4.6+
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Acked-by: Vladimir Davydov <vdavydov.dev@...il.com>

> ---
>  mm/workingset.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 617475f529f4..fb1f9183d89a 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -348,7 +348,7 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
>  	shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
>  	local_irq_enable();
>  
> -	if (memcg_kmem_enabled()) {
> +	if (sc->memcg) {
>  		pages = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
>  						     LRU_ALL_FILE);
>  	} else {
> -- 
> 2.10.2
>