Re: [PATCH] mm: memcontrol: do not recurse in direct reclaim

[Michal Hocko <mhocko@kernel.org>]

On Tue 25-10-16 10:10:50, Johannes Weiner wrote:
> On Tue, Oct 25, 2016 at 11:07:47AM +0200, Michal Hocko wrote:
> > Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Thank you.
> 
> > I would prefer to have the PF_MEMALLOC condition in a check on its own
> > with a short explanation that we really do not want to recurse to the
> > reclaim due to stack overflows.
> 
> Okay, fair enough. I also added why we prefer temporarily breaching
> the limit over failing the allocation. How is this?
> 
> >From 9034d2e6a21036774df9a8e021511720cf432c82 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Mon, 24 Oct 2016 16:01:55 -0400
> Subject: [PATCH] mm: memcontrol: do not recurse in direct reclaim
> 
> On 4.0, we saw a stack corruption from a page fault entering direct
> memory cgroup reclaim, calling into btrfs_releasepage(), which then
> tried to allocate an extent and recursed back into a kmem charge ad
> nauseam:
> 
> [...]
> [<ffffffff8136590c>] btrfs_releasepage+0x2c/0x30
> [<ffffffff811559a2>] try_to_release_page+0x32/0x50
> [<ffffffff81168cea>] shrink_page_list+0x6da/0x7a0
> [<ffffffff811693b5>] shrink_inactive_list+0x1e5/0x510
> [<ffffffff8116a0a5>] shrink_lruvec+0x605/0x7f0
> [<ffffffff8116a37e>] shrink_zone+0xee/0x320
> [<ffffffff8116a934>] do_try_to_free_pages+0x174/0x440
> [<ffffffff8116adf7>] try_to_free_mem_cgroup_pages+0xa7/0x130
> [<ffffffff811b738b>] try_charge+0x17b/0x830
> [<ffffffff811bb5b0>] memcg_charge_kmem+0x40/0x80
> [<ffffffff811a96a9>] new_slab+0x2d9/0x5a0
> [<ffffffff817b2547>] __slab_alloc+0x2fd/0x44f
> [<ffffffff811a9b03>] kmem_cache_alloc+0x193/0x1e0
> [<ffffffff813801e1>] alloc_extent_state+0x21/0xc0
> [<ffffffff813820c5>] __clear_extent_bit+0x2b5/0x400
> [<ffffffff81386d03>] try_release_extent_mapping+0x1a3/0x220
> [<ffffffff813658a1>] __btrfs_releasepage+0x31/0x70
> [<ffffffff8136590c>] btrfs_releasepage+0x2c/0x30
> [<ffffffff811559a2>] try_to_release_page+0x32/0x50
> [<ffffffff81168cea>] shrink_page_list+0x6da/0x7a0
> [<ffffffff811693b5>] shrink_inactive_list+0x1e5/0x510
> [<ffffffff8116a0a5>] shrink_lruvec+0x605/0x7f0
> [<ffffffff8116a37e>] shrink_zone+0xee/0x320
> [<ffffffff8116a934>] do_try_to_free_pages+0x174/0x440
> [<ffffffff8116adf7>] try_to_free_mem_cgroup_pages+0xa7/0x130
> [<ffffffff811b738b>] try_charge+0x17b/0x830
> [<ffffffff811bbfd5>] mem_cgroup_try_charge+0x65/0x1c0
> [<ffffffff8118338f>] handle_mm_fault+0x117f/0x1510
> [<ffffffff81041cf7>] __do_page_fault+0x177/0x420
> [<ffffffff81041fac>] do_page_fault+0xc/0x10
> [<ffffffff817c0182>] page_fault+0x22/0x30
> 
> On later kernels, kmem charging is opt-in rather than opt-out, and
> that particular kmem allocation in btrfs_releasepage() is no longer
> being charged and won't recurse and overrun the stack anymore. But
> it's not impossible for an accounted allocation to happen from the
> memcg direct reclaim context, and we needed to reproduce this crash
> many times before we even got a useful stack trace out of it.
> 
> Like other direct reclaimers, mark tasks in memcg reclaim PF_MEMALLOC
> to avoid recursing into any other form of direct reclaim. Then let
> recursive charges from PF_MEMALLOC contexts bypass the cgroup limit.
> 

Should we mark this for stable (up to 4.5) which changed the out-out to
opt-in?

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/memcontrol.c | 9 +++++++++
>  mm/vmscan.c     | 2 ++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ae052b5e3315..0f870ba43942 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1917,6 +1917,15 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  		     current->flags & PF_EXITING))
>  		goto force;
>  
> +	/*
> +	 * Prevent unbounded recursion when reclaim operations need to
> +	 * allocate memory. This might exceed the limits temporarily,
> +	 * but we prefer facilitating memory reclaim and getting back
> +	 * under the limit over triggering OOM kills in these cases.
> +	 */
> +	if (unlikely(current->flags & PF_MEMALLOC))
> +		goto force;
> +

OK, sounds good to me. THanks!

>  	if (unlikely(task_in_memcg_oom(current)))
>  		goto nomem;
>  
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 744f926af442..76fda2268148 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3043,7 +3043,9 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>  					    sc.gfp_mask,
>  					    sc.reclaim_idx);
>  
> +	current->flags |= PF_MEMALLOC;
>  	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
> +	current->flags &= ~PF_MEMALLOC;
>  
>  	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
>  
> -- 
> 2.10.0

-- 
Michal Hocko
SUSE Labs