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Date: Mon, 17 Mar 2014 23:20:57 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Sasha Levin <sasha.levin@...cle.com>, Bob Liu <bob.liu@...cle.com>
CC: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
Rik van Riel <riel@...hat.com>, joern@...fs.org,
Michel Lespinasse <walken@...gle.com>
Subject: Re: [PATCH 1/3] mm: munlock: fix a bug where THP tail page is encountered
On 17.3.2014 22:08, Sasha Levin wrote:
> On 03/17/2014 08:38 AM, Vlastimil Babka wrote:
>> On 03/15/2014 04:06 AM, Sasha Levin wrote:
>>> On 03/14/2014 07:55 PM, Sasha Levin wrote:
>>>> On 12/17/2013 08:00 AM, Vlastimil Babka wrote:
>>>>> From: Vlastimil Babka<vbabka@...e.cz>
>>>>> Date: Fri, 13 Dec 2013 14:25:21 +0100
>>>>> Subject: [PATCH 1/3] mm: munlock: fix a bug where THP tail page is
>>>>> encountered
>>>>>
>>>>> Since commit ff6a6da60 ("mm: accelerate munlock() treatment of THP
>>>>> pages")
>>>>> munlock skips tail pages of a munlocked THP page. However, when
>>>>> the head page
>>>>> already has PageMlocked unset, it will not skip the tail pages.
>>>>>
>>>>> Commit 7225522bb ("mm: munlock: batch non-THP page isolation and
>>>>> munlock+putback using pagevec") has added a PageTransHuge() check
>>>>> which
>>>>> contains VM_BUG_ON(PageTail(page)). Sasha Levin found this
>>>>> triggered using
>>>>> trinity, on the first tail page of a THP page without PageMlocked
>>>>> flag.
>>>>>
>>>>> This patch fixes the issue by skipping tail pages also in the case
>>>>> when
>>>>> PageMlocked flag is unset. There is still a possibility of race
>>>>> with THP page
>>>>> split between clearing PageMlocked and determining how many pages
>>>>> to skip.
>>>>> The race might result in former tail pages not being skipped,
>>>>> which is however
>>>>> no longer a bug, as during the skip the PageTail flags are cleared.
>>>>>
>>>>> However this race also affects correctness of NR_MLOCK accounting,
>>>>> which is to
>>>>> be fixed in a separate patch.
>>>>
>>>> I've hit the same thing again, on the latest -next, this time with
>>>> a different trace:
>>>>
>>>> [ 539.199120] page:ffffea0013249a80 count:0 mapcount:1
>>>> mapping: (null) index:0x0
>>>> [ 539.200429] page flags: 0x12fffff80008000(tail)
>>>> [ 539.201167] ------------[ cut here ]------------
>>>> [ 539.201889] kernel BUG at include/linux/page-flags.h:415!
>>>> [ 539.202859] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>>>> [ 539.204588] Dumping ftrace buffer:
>>>> [ 539.206415] (ftrace buffer empty)
>>>> [ 539.207022] Modules linked in:
>>>> [ 539.207503] CPU: 3 PID: 18262 Comm: trinity-c228 Tainted:
>>>> G W 3.14.0-rc6-next-20140313-sasha-00010-gb8c1db1-dirty #217
>>>> [ 539.209012] task: ffff880627b10000 ti: ffff8805a44c2000 task.ti:
>>>> ffff8805a44c2000
>>>> [ 539.209989] RIP: munlock_vma_pages_range+0x93/0x1d0
>>>> (include/linux/page-flags.h:415 mm/mlock.c:494)
>>>> [ 539.210263] RSP: 0000:ffff8805a44c3e08 EFLAGS: 00010246
>>>> [ 539.210263] RAX: ffff88052ae126a0 RBX: 000000000006a000 RCX:
>>>> 0000000000000099
>>>> [ 539.210263] RDX: 0000000000000000 RSI: ffff880627b10cf0 RDI:
>>>> 0000000004c926a0
>>>> [ 539.210263] RBP: ffff8805a44c3ec8 R08: 0000000000000001 R09:
>>>> 0000000000000001
>>>> [ 539.210263] R10: 0000000000000001 R11: 0000000000000001 R12:
>>>> ffffea0013249a80
>>>> [ 539.210263] R13: ffff88039dc95a00 R14: 000000000006b000 R15:
>>>> ffff8805a44c3e94
>>>> [ 539.210263] FS: 00007fd6ce14a700(0000)
>>>> GS:ffff88042b800000(0000) knlGS:0000000000000000
>>>> [ 539.210263] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>>>> [ 539.210263] CR2: 00007fd6ce0ef6ac CR3: 00000006025cd000 CR4:
>>>> 00000000000006a0
>>>> [ 539.210263] DR0: 0000000000698000 DR1: 0000000000000000 DR2:
>>>> 0000000000000000
>>>> [ 539.210263] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
>>>> 0000000000000600
>>>> [ 539.210263] Stack:
>>>> [ 539.210263] 0000000000000000 0000000000000000 00018805a44c3e38
>>>> 0000000000000000
>>>> [ 539.210263] 0000000000000000 ffff88039dc95a00 00000000a44c3e88
>>>> 0000000000000000
>>>> [ 539.210263] 00ff8805a44c3e58 ffff880528f0a0f0 ffff8805a44c3eb8
>>>> ffff88039dc95a00
>>>> [ 539.210263] Call Trace:
>>>> [ 539.210263] do_munmap+0x1d2/0x360 (mm/internal.h:168
>>>> mm/mmap.c:2547)
>>>> [ 539.210263] ? down_write+0xa6/0xc0 (kernel/locking/rwsem.c:51)
>>>> [ 539.210263] ? vm_munmap+0x46/0x80 (mm/mmap.c:2571)
>>>> [ 539.210263] vm_munmap+0x54/0x80 (mm/mmap.c:2572)
>>>> [ 539.210263] SyS_munmap+0x2c/0x40 (mm/mmap.c:2577)
>>>> [ 539.210263] tracesys+0xdd/0xe2 (arch/x86/kernel/entry_64.S:749)
>>>> [ 539.210263] Code: ff 49 89 c4 48 85 c0 0f 84 f3 00 00 00 48 3d
>>>> 00 f0 ff ff 0f 87 e7 00 00 00 48 8b 00 66 85 c0 79 17 31 f6 4c 89
>>>> e7 e8 4d d2 fc ff <0f> 0b 0f 1f 00 eb fe 66 0f 1f 44 00 00 49 8b 04
>>>> 24 f6 c4 40 74
>>>> [ 539.210263] RIP munlock_vma_pages_range+0x93/0x1d0
>>>> (include/linux/page-flags.h:415 mm/mlock.c:494)
>>>> [ 539.210263] RSP <ffff8805a44c3e08>
>>>> [ 539.236666] ---[ end trace 4e90dc9141579181 ]---
>>>>
>>>>
>>>> Thanks,
>>>> Sasha
>>>
>>> And another related trace:
>>>
>>> [ 741.192502] kernel BUG at mm/mlock.c:528!
>>> [ 741.193088] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>>> [ 741.194177] Dumping ftrace buffer:
>>> [ 741.194645] (ftrace buffer empty)
>>> [ 741.195109] Modules linked in:
>>> [ 741.195728] CPU: 23 PID: 19908 Comm: trinity-c264 Tainted:
>>> G W 3.14.0-rc6-next-20140314-sasha-00012-g5590866 #219
>>> [ 741.197549] task: ffff88061fc2b000 ti: ffff8805decb8000 task.ti:
>>> ffff8805decb8000
>>> [ 741.198548] RIP: munlock_vma_pages_range+0x176/0x1d0
>>> (mm/mlock.c:528)
>>> [ 741.199754] RSP: 0018:ffff8805decb9e08 EFLAGS: 00010206
>>> [ 741.200085] RAX: 00000000000001ff RBX: 0000000000111000 RCX:
>>> 0000000000000000
>>> [ 741.200085] RDX: 0000000000000111 RSI: ffffffff81295fdd RDI:
>>> ffffffff84490705
>>> [ 741.200085] RBP: ffff8805decb9ec8 R08: 0000000000000000 R09:
>>> 0000000000000000
>>> [ 741.200085] R10: 0000000000000001 R11: 0000000000000000 R12:
>>> fffffffffffffff2
>>> [ 741.200085] R13: ffff880221044e00 R14: 0000000000112000 R15:
>>> ffff8805decb9e94
>>> [ 741.200085] FS: 00007f4bec6bc700(0000) GS:ffff88082ba00000(0000)
>>> knlGS:0000000000000000
>>> [ 741.200085] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>>> [ 741.200085] CR2: 0000000003109a98 CR3: 00000005decaf000 CR4:
>>> 00000000000006a0
>>> [ 741.200085] Stack:
>>> [ 741.200085] 0000000000000000 0000000000000000 00018805decb9e38
>>> 0000000000000000
>>> [ 741.200085] 0000000000000000 ffff880221044e00 00000000decb9e88
>>> 0000000000000000
>>> [ 741.200085] 00ff8805decb9e58 ffff880ff376f450 ffff8805decb9eb8
>>> ffff880221044e00
>>> [ 741.200085] Call Trace:
>>> [ 741.200085] do_munmap+0x1d2/0x360 (mm/internal.h:168
>>> mm/mmap.c:2547)
>>> [ 741.200085] ? down_write+0xa6/0xc0 (kernel/locking/rwsem.c:51)
>>> [ 741.200085] ? vm_munmap+0x46/0x80 (mm/mmap.c:2571)
>>> [ 741.200085] vm_munmap+0x54/0x80 (mm/mmap.c:2572)
>>> [ 741.200085] SyS_munmap+0x2c/0x40 (mm/mmap.c:2577)
>>> [ 741.200085] tracesys+0xdd/0xe2 (arch/x86/kernel/entry_64.S:749)
>>> [ 741.200085] Code: fd ff ff 4c 89 e6 48 89 c3 48 8d bd 40 ff ff ff
>>> e8 80 fa ff ff eb 2f 66 0f 1f 44 00 00 8b 45 cc 48 89 da 48 c1 ea 0c
>>> 85 d0 74 12 <0f> 0b 0f 1f 84 00 00 00 00 00 eb fe 66 0f 1f 44 00 00
>>> ff c0 48
>>> [ 741.200085] RIP munlock_vma_pages_range+0x176/0x1d0
>>> (mm/mlock.c:528)
>>> [ 741.200085] RSP <ffff8805decb9e08>
>>
>> Sigh, again? I really wonder what is it this time :)
>> Please try this debug patch, hopefully we learn something:
>>
>> diff --git a/mm/mlock.c b/mm/mlock.c
>> index 4e1a6816..7094bac 100644
>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -469,12 +469,14 @@ static unsigned long
>> __munlock_pagevec_fill(struct pagevec *pvec,
>> void munlock_vma_pages_range(struct vm_area_struct *vma,
>> unsigned long start, unsigned long end)
>> {
>> + unsigned long orig_start = start;
>> + unsigned long page_increm = 0;
>> +
>> vma->vm_flags &= ~VM_LOCKED;
>>
>> while (start < end) {
>> struct page *page = NULL;
>> unsigned int page_mask;
>> - unsigned long page_increm;
>> struct pagevec pvec;
>> struct zone *zone;
>> int zoneid;
>> @@ -491,6 +493,24 @@ void munlock_vma_pages_range(struct
>> vm_area_struct *vma,
>> &page_mask);
>>
>> if (page && !IS_ERR(page)) {
>> + if (PageTail(page)) {
>> + struct page *first_page;
>> + dump_page(page, "Unexpected PageTail");
>> + printk("start=%lu pfn=%lu orig_start=%lu "
>> + "page_increm=%lu "
>> + "vm_start=%lu vm_end=%lu vm_flags=%lu\n",
>> + start, page_to_pfn(page), orig_start,
>> + page_increm,
>> + vma->vm_start, vma->vm_end,
>> + vma->vm_flags);
>> + first_page = page->first_page;
>> + printk("first_page pfn=%lu\n",
>> + page_to_pfn(first_page));
>> + dump_page(first_page, "first_page of unexpected
>> PageTail page");
>> + if (PageCompound(first_page))
>> + printk("first_page is compound with order=%d\n",
>> compound_order(first_page));
>> + VM_BUG_ON(true);
>> + }
>> if (PageTransHuge(page)) {
>> lock_page(page);
>> /*
>> @@ -525,7 +545,25 @@ void munlock_vma_pages_range(struct
>> vm_area_struct *vma,
>> }
>> }
>> /* It's a bug to munlock in the middle of a THP page */
>> - VM_BUG_ON((start >> PAGE_SHIFT) & page_mask);
>> + if ((start >> PAGE_SHIFT) & page_mask) {
>> + dump_page(page, "unexpected middle of THP page");
>> + printk("start=%lu pfn=%lu orig_start=%lu "
>
> In this scenario page might be error or NULL, no?
>
Hm I doubt page_mask would be nonzero in such case, but who knows what
weirdness causes this...
Feel free to move the dump_page() below printk() so we still get useful
info before it crashes on
dump_page() in such situation.
>
> Thanks,
> Sasha
>
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