Re: memcg uncharge page counter mismatch

From: Minchan Kim <minchan@kernel.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: memcg uncharge page counter mismatch
Date: Thu, 3 Dec 2015 10:34:04 +0900	[thread overview]
Message-ID: <20151203013404.GA30779@bbox> (raw)
In-Reply-To: <20151202101643.GC25284@dhcp22.suse.cz>

On Wed, Dec 02, 2015 at 11:16:43AM +0100, Michal Hocko wrote:
> On Tue 01-12-15 22:34:55, Minchan Kim wrote:
> > With new test on mmotm-2015-11-25-17-08, I saw below WARNING message
> > several times. I couldn't see it with reverting new THP refcount
> > redesign.
> 
> Just a wild guess. What prevents migration/compaction from calling
> split_huge_page on thp zero page? There is VM_BUG_ON but it is not clear

I guess migration should work with LRU pages now but zero page couldn't
stay there.

> whether you run with CONFIG_DEBUG_VM enabled.

I enabled VM_DEBUG_VM.

> 
> Also, how big is the underflow?

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 7c6a63d..adc27c3 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -23,6 +23,8 @@ void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
 
        new = atomic_long_sub_return(nr_pages, &counter->count);
        /* More uncharges than charges? */
+       if (new < 0)
+               printk("nr_pages %lu new %ld\n", nr_pages, new);
        WARN_ON_ONCE(new < 0);
 }

nr_pages 512 new -31
------------[ cut here ]------------
WARNING: CPU: 3 PID: 1145 at mm/page_counter.c:28 page_counter_cancel+0x44/0x50()
Modules linked in:
CPU: 3 PID: 1145 Comm: madvise_test Not tainted 4.4.0-rc2-mm1-kirill+ #17
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffffffff81782f09 ffff8800598c3b90 ffffffff8126f476 0000000000000000
 ffff8800598c3bc8 ffffffff8103e476 ffff88007f14f8b0 0000000000000200
 0000000000000000 0000000000000200 ffff88007f14f800 ffff8800598c3bd8
Call Trace:
 [<ffffffff8126f476>] dump_stack+0x44/0x5e
 [<ffffffff8103e476>] warn_slowpath_common+0x86/0xc0
 [<ffffffff8103e56a>] warn_slowpath_null+0x1a/0x20
 [<ffffffff8114c744>] page_counter_cancel+0x44/0x50
 [<ffffffff8114c842>] page_counter_uncharge+0x22/0x30
 [<ffffffff8114fe07>] uncharge_batch+0x47/0x140
 [<ffffffff81150023>] uncharge_list+0x123/0x190
 [<ffffffff811521f9>] mem_cgroup_uncharge+0x29/0x30
 [<ffffffff810fe3be>] __page_cache_release+0x15e/0x200
 [<ffffffff810fe47e>] __put_compound_page+0x1e/0x50
 [<ffffffff810fe960>] release_pages+0xd0/0x370
 [<ffffffff8113042d>] free_pages_and_swap_cache+0x9d/0x120
 [<ffffffff8111a516>] tlb_flush_mmu_free+0x36/0x60
 [<ffffffff8111b60c>] tlb_finish_mmu+0x1c/0x50
 [<ffffffff81125f1c>] exit_mmap+0xec/0x140
 [<ffffffff8103bd56>] mmput+0x56/0xe0
 [<ffffffff8103ff4d>] do_exit+0x1fd/0xaa0
 [<ffffffff8104086f>] do_group_exit+0x3f/0xb0
 [<ffffffff810408f4>] SyS_exit_group+0x14/0x20
 [<ffffffff8142b617>] entry_SYSCALL_64_fastpath+0x12/0x6a
---[ end trace 872ed93351e964c0 ]---
nr_pages 293 new -324
nr_pages 16 new -340
nr_pages 342 new -91
nr_pages 246 new -337
nr_pages 15 new -352
nr_pages 15 new -367
