Re: [PATCH 2/2] mm/page_alloc: DEBUG_VM checks for free_list placement of CMA and RESERVE pages

[Vlastimil Babka <vbabka@suse.cz>]

On 05/05/2014 04:36 PM, Sasha Levin wrote:
> On 05/02/2014 08:08 AM, Vlastimil Babka wrote:
>> On 04/30/2014 11:46 PM, Sasha Levin wrote:
>>>> On 04/03/2014 11:40 AM, Vlastimil Babka wrote:
>>>>>> For the MIGRATE_RESERVE pages, it is important they do not get misplaced
>>>>>> on free_list of other migratetype, otherwise the whole MIGRATE_RESERVE
>>>>>> pageblock might be changed to other migratetype in try_to_steal_freepages().
>>>>>> For MIGRATE_CMA, the pages also must not go to a different free_list, otherwise
>>>>>> they could get allocated as unmovable and result in CMA failure.
>>>>>>
>>>>>> This is ensured by setting the freepage_migratetype appropriately when placing
>>>>>> pages on pcp lists, and using the information when releasing them back to
>>>>>> free_list. It is also assumed that CMA and RESERVE pageblocks are created only
>>>>>> in the init phase. This patch adds DEBUG_VM checks to catch any regressions
>>>>>> introduced for this invariant.
>>>>>>
>>>>>> Cc: Yong-Taek Lee <ytk.lee@samsung.com>
>>>>>> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
>>>>>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>>>>> Cc: Mel Gorman <mgorman@suse.de>
>>>>>> Cc: Minchan Kim <minchan@kernel.org>
>>>>>> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>>>>>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>>>>>> Cc: Hugh Dickins <hughd@google.com>
>>>>>> Cc: Rik van Riel <riel@redhat.com>
>>>>>> Cc: Michal Nazarewicz <mina86@mina86.com>
>>>>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>>>>
>>>> Two issues with this patch.
>>>>
>>>> First:
>>>>
>>>> [ 3446.320082] kernel BUG at mm/page_alloc.c:1197!
>>>> [ 3446.320082] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>>>> [ 3446.320082] Dumping ftrace buffer:
>>>> [ 3446.320082]    (ftrace buffer empty)
>>>> [ 3446.320082] Modules linked in:
>>>> [ 3446.320082] CPU: 1 PID: 8923 Comm: trinity-c42 Not tainted 3.15.0-rc3-next-20140429-sasha-00015-g7c7e0a7-dirty #427
>>>> [ 3446.320082] task: ffff88053e208000 ti: ffff88053e246000 task.ti: ffff88053e246000
>>>> [ 3446.320082] RIP: get_page_from_freelist (mm/page_alloc.c:1197 mm/page_alloc.c:1548 mm/page_alloc.c:2036)
>>>> [ 3446.320082] RSP: 0018:ffff88053e247778  EFLAGS: 00010002
>>>> [ 3446.320082] RAX: 0000000000000003 RBX: ffffea0000f40000 RCX: 0000000000000008
>>>> [ 3446.320082] RDX: 0000000000000002 RSI: 0000000000000003 RDI: 00000000000000a0
>>>> [ 3446.320082] RBP: ffff88053e247868 R08: 0000000000000007 R09: 0000000000000000
>>>> [ 3446.320082] R10: ffff88006ffcef00 R11: 0000000000000000 R12: 0000000000000014
>>>> [ 3446.335888] R13: ffffea000115ffe0 R14: ffffea000115ffe0 R15: 0000000000000000
>>>> [ 3446.335888] FS:  00007f8c9f059700(0000) GS:ffff88006ec00000(0000) knlGS:0000000000000000
>>>> [ 3446.335888] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>>>> [ 3446.335888] CR2: 0000000002cbc048 CR3: 000000054cdb4000 CR4: 00000000000006a0
>>>> [ 3446.335888] DR0: 00000000006de000 DR1: 00000000006de000 DR2: 0000000000000000
>>>> [ 3446.335888] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000602
>>>> [ 3446.335888] Stack:
>>>> [ 3446.335888]  ffff88053e247798 ffff88006eddc0b8 0000000000000016 0000000000000000
>>>> [ 3446.335888]  ffff88006ffd2068 ffff88006ffdb008 0000000100000000 0000000000000000
>>>> [ 3446.335888]  ffff88006ffdb000 0000000000000000 0000000000000003 0000000000000001
>>>> [ 3446.335888] Call Trace:
>>>> [ 3446.335888] __alloc_pages_nodemask (mm/page_alloc.c:2731)
>>>> [ 3446.335888] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
>>>> [ 3446.335888] alloc_pages_vma (include/linux/mempolicy.h:76 mm/mempolicy.c:1998)
>>>> [ 3446.335888] ? shmem_alloc_page (mm/shmem.c:881)
>>>> [ 3446.335888] ? kvm_clock_read (arch/x86/include/asm/preempt.h:90 arch/x86/kernel/kvmclock.c:86)
>>>> [ 3446.335888] shmem_alloc_page (mm/shmem.c:881)
>>>> [ 3446.335888] ? __const_udelay (arch/x86/lib/delay.c:126)
>>>> [ 3446.335888] ? __rcu_read_unlock (kernel/rcu/update.c:97)
>>>> [ 3446.335888] ? find_get_entry (mm/filemap.c:979)
>>>> [ 3446.335888] ? find_get_entry (mm/filemap.c:940)
>>>> [ 3446.335888] ? find_lock_entry (mm/filemap.c:1024)
>>>> [ 3446.335888] shmem_getpage_gfp (mm/shmem.c:1130)
>>>> [ 3446.335888] ? sched_clock_local (kernel/sched/clock.c:214)
>>>> [ 3446.335888] ? do_read_fault.isra.42 (mm/memory.c:3523)
>>>> [ 3446.335888] shmem_fault (mm/shmem.c:1237)
>>>> [ 3446.335888] ? do_read_fault.isra.42 (mm/memory.c:3523)
>>>> [ 3446.335888] __do_fault (mm/memory.c:3344)
>>>> [ 3446.335888] ? _raw_spin_unlock (arch/x86/include/asm/preempt.h:98 include/linux/spinlock_api_smp.h:152 kernel/locking/spinlock.c:183)
>>>> [ 3446.335888] do_read_fault.isra.42 (mm/memory.c:3524)
>>>> [ 3446.335888] ? get_parent_ip (kernel/sched/core.c:2485)
>>>> [ 3446.335888] ? get_parent_ip (kernel/sched/core.c:2485)
>>>> [ 3446.335888] __handle_mm_fault (mm/memory.c:3662 mm/memory.c:3823 mm/memory.c:3950)
>>>> [ 3446.335888] ? __const_udelay (arch/x86/lib/delay.c:126)
>>>> [ 3446.335888] ? __rcu_read_unlock (kernel/rcu/update.c:97)
>>>> [ 3446.335888] handle_mm_fault (mm/memory.c:3973)
>>>> [ 3446.335888] __get_user_pages (mm/memory.c:1863)
>>>> [ 3446.335888] ? preempt_count_sub (kernel/sched/core.c:2541)
>>>> [ 3446.335888] __mlock_vma_pages_range (mm/mlock.c:255)
>>>> [ 3446.335888] __mm_populate (mm/mlock.c:711)
>>>> [ 3446.335888] vm_mmap_pgoff (include/linux/mm.h:1841 mm/util.c:402)
>>>> [ 3446.335888] SyS_mmap_pgoff (mm/mmap.c:1378)
>>>> [ 3446.335888] ? syscall_trace_enter (include/linux/context_tracking.h:27 arch/x86/kernel/ptrace.c:1461)
>>>> [ 3446.335888] ia32_do_call (arch/x86/ia32/ia32entry.S:430)
>>>> [ 3446.335888] Code: 00 66 0f 1f 44 00 00 ba 02 00 00 00 31 f6 48 89 c7 e8 c1 c3 ff ff 48 8b 53 10 83 f8 03 74 08 83 f8 04 75 13 0f 1f 00 39 d0 74 0c <0f> 0b 66 2e 0f 1f 84 00 00 00 00 00 45 85 ff 75 15 49 8b 55 00
>>>> [ 3446.335888] RIP get_page_from_freelist (mm/page_alloc.c:1197 mm/page_alloc.c:1548 mm/page_alloc.c:2036)
>>>> [ 3446.335888]  RSP <ffff88053e247778>
>> Hey, that's not an issue, that means the patch works as intended :) And
>> I believe it's not a bug introduced by PATCH 1/2.
>>
>> So, according to my decodecode reading, RAX is the results of
>> get_pageblock_migratetype() and it's MIGRATE_RESERVE. RDX is the result
>> of get_freepage_migratetype() and it's MIGRATE_UNMOVABLE. The
>> freepage_migratetype has just been set either by __rmqueue_smallest() or
>> __rmqueue_fallback(), according to the free_list the page has been taken
>> from. So this looks like a page from MIGRATE_RESERVE pageblock found on
>> the !MIGRATE_RESERVE free_list, which is exactly what the patch intends
>> to catch.
>>
>> I think there are two possible explanations.
>>
>> 1) the pageblock is genuinely MIGRATE_RESERVE and it was misplaced by
>> mistake. I think it wasn't in free_pcppages_bulk() as there's the same
>> VM_BUG_ON which would supposedly trigger at the moment of displacing. In
>> theory it's possible that there's a race through __free_pages_ok() ->
>> free_one_page() where the get_pageblock_migratetype() in
>> __free_pages_ok() would race with set_pageblock_migratetype() and result
>> in bogus value. But nobody should be calling set_pageblock_migratetype()
>> on a MIGRATE_RESERVE pageblock.
>>
>> 2) the pageblock was marked as MIGRATE_RESERVE due to a race between
>> set_pageblock_migratetype() and set_pageblock_skip(). The latter is
>> currently not serialized by zone->lock, nor it uses atomic bit set. So
>> it may result in lost updates in a racing set_pageblock_migratetype(). I
>> think a well-placed race when changing pageblock from MIGRATE_MOVABLE to
>> MIGRATE_RECLAIMABLE could result in MIGRATE_RESERVE value. Similar races
>> have been already observed to be a problem where frequent changing
>> to/from MIGRATE_ISOLATE is involved, and I did a patch series to address
>> this, but it was not complete and I postponed it after Mel's changes
>> that remove the racy for-cycles completely. So it might be that his
>> "[PATCH 08/17] mm: page_alloc: Use word-based accesses for get/set
>> pageblock bitmaps" already solves this bug (but maybe only on certain
>> architectures where you don't need atomic operations). You might try
>> that patch if you can reproduce this bug frequently enough?
>
> I've tried that patch, but still see the same BUG_ON.

Oh damn, I've realized that my assumptions about MIGRATE_RESERVE 
pageblocks being created only on zone init time were wrong. 
setup_zone_migrate_reserve() is called also from the handler of 
min_free_kbytes sysctl... does trinity try to change that while running?
The function will change MOVABLE pageblocks to RESERVE and try to move 
all free pages to the RESERVE free_list, but of course pages on pcplists 
will remain MOVABLE and may trigger the VM_BUG_ON. You triggered the bug 
with page on MOVABLE free_list (in the first reply I said its UNMOVABLE 
by mistake) so this might be good explanation if trinity changes 
min_free_kbytes.

Furthermore, I think there's a problem that setup_zone_migrate_reserve() 
operates on pageblocks, but as MAX_ODER is higher than pageblock_order, 
RESERVE pages might be merged with buddies of different migratetype and 
end up on their free_list. That seems to me like a flaw in the design of 
reserves, but perhaps others won't think it's serious enough to fix?

So in the end this VM_DEBUG check probably cannot work anymore for 
MIGRATE_RESERVE, only for CMA. I'm not sure if it's worth keeping it 
only for CMA, what are the CMA guys' opinions on that?

Also this means that the 1/2 patch "prevent MIGRATE_RESERVE pages from 
being misplaced" still won't prevent stealing a MIGRATE_RESERVE 
pageblock when __rmqueue_fallback() encounters a strayed MIGRATE_RESERVE 
page on e.g. a MOVABLE freelist. This is fixable by having 
__rmqueue_fallback() not trusting the migratetype of free_list and 
checking for pageblock_migratetype. I hate that, but at least it's not 
on the fast path...

> Thanks,
> Sasha
>