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

[Joonsoo Kim]

On Wed, May 07, 2014 at 04:59:07PM +0200, Vlastimil Babka wrote:
> On 05/07/2014 03:33 AM, Minchan Kim wrote:
> > On Mon, May 05, 2014 at 05:50:46PM +0200, Vlastimil Babka wrote:
> >> 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@xxxxxxxxxxx>
> >>>>>>>> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> >>>>>>>> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >>>>>>>> Cc: Mel Gorman <mgorman@xxxxxxx>
> >>>>>>>> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> >>>>>>>> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> >>>>>>>> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> >>>>>>>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> >>>>>>>> Cc: Rik van Riel <riel@xxxxxxxxxx>
> >>>>>>>> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> >>>>>>>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> >>>>>>
> >>>>>> 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?

I wanna know who want MIGRATE_RESERVE. On my previous testing, one
pageblock for MIGRATE_RESERVE is merged with buddies of different
migratetype during boot-up and never come back again. But my system works
well. :)

If it is really useful feature, we can fix the situation by aligning
reserve size and pfn to MAX_ORDER_NR_PAGES.

And, IMHO, it isn't reasonable to increase, decrease and change
MIGRATE_RESERVE pageblock by the handler of min_free_kbytes sysctl,
because new pageblock may have no free pages to reserve. I think that
it is better to prevent sysctl handler from changing MIGRATE_RESERVE
pageblock, after initialization.

In conclusion, if MIGRATE_RESERVE is useful enough to maintain, to fix
above problem and keep this patch is preferable to me.

> >>
> >> 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?
> > 
> > I really don't want it. That was I didn't add my Acked-by at that time.
> > For a long time, I never wanted to add more overhead hot path due to
> > CMA unless it's really critical. It's same to this.
> > Although such debug patch helps to notice something goes wrong for CMA,
> > more information would be helpful to know why CMA failed because
> > there are another potential reasons to fail CMA allocation.
> > 
> > One of the idea about that is to store alloc trace into somewhere(ex,
> > naive idea is page description like page-owner) and then we could investigate
> > what's the owner of that page so we could know why we fail to migrate it out.
> > With that, we would figure out how on earth such page is allocated from CMA
> > and it would be more helpful rather just VM_BUG_ON notice.
> > 
> > The whole point is I'd like to avoid adding more overhead to hot path for
> > rare case although it's debugging feature.
> 
> OK. I'm not that concerned with VM_DEBUG overhead as it's intended for
> testing, not production. But as you say the patch is not that useful
> without the MIGRATE_RESERVE part, so Andrew could you please drop the
> patch (mm-page_alloc-debug_vm-checks-for-free_list-placement-of-cma-and-reserve-pages.patch)?

I also think that VM_DEBUG overhead isn't problem because of same
reason from Vlastimil.

Thanks.
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