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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c | 9 +++++----
 mm/vmscan.c     | 2 ++
 2 files changed, 7 insertions(+), 4 deletions(-)

Hey guys, can anyone think of a reason why this might not be a good
idea? We've never really needed this in the past because page reclaim
doesn't recurse into instantiating another LRU page, especially with
GFP_NOFS. But with a wider variety of tracked allocations, it's no
longer that obvious. It seems like a risky hole to leave around.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ae052b5e3315..3dac6f4ba4cf 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1908,13 +1908,14 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 
 	/*
 	 * Unlike in global OOM situations, memcg is not in a physical
-	 * memory shortage.  Allow dying and OOM-killed tasks to
-	 * bypass the last charges so that they can exit quickly and
-	 * free their memory.
+	 * memory shortage. Allow dying and OOM-killed tasks to bypass
+	 * the last charges so that they can exit quickly and free
+	 * their memory. The same applies for recursing reclaimers.
 	 */
 	if (unlikely(test_thread_flag(TIF_MEMDIE) ||
 		     fatal_signal_pending(current) ||
-		     current->flags & PF_EXITING))
+		     current->flags & PF_EXITING ||
+		     current->flags & PF_MEMALLOC))
 		goto force;
 
 	if (unlikely(task_in_memcg_oom(current)))
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