Re: [PATCH 1/3] mm: munlock: fix a bug where THP tail page is encountered

From: Vlastimil Babka
Date: Mon Mar 17 2014 - 08:38:59 EST

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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@xxxxxxx>
>>> 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 "
+ "prev_page_increm=%lu page_mask=%u "
+ "vm_start=%lu vm_end=%lu vm_flags=%lu\n",
+ start, page_to_pfn(page), orig_start,
+ page_increm, page_mask,
+ vma->vm_start, vma->vm_end,
+ vma->vm_flags);
+ if (PageTail(page)) {
+ struct page *first_page = page->first_page;
+ printk("first_page pfn=%lu\n",
+ page_to_pfn(first_page));
+ dump_page(first_page, "first_page of unexpected middle of THP page");
+ if (PageCompound(first_page))
+ printk("first_page is compound with order=%d\n", compound_order(first_page));
+ }
+ VM_BUG_ON(true);
+ }
page_increm = 1 + page_mask;
start += page_increm * PAGE_SIZE;
next:

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Next message: Chanwoo Choi: "[RFC PATCH 3/4] Documentation: dt: charger-manager: Change the way to get fuel-gauge power-supply device"
Previous message: Tetsuo Handa: "Re: [v3.13][v3.14][Regression] kthread: make kthread_create()killable"
Next in thread: Sasha Levin: "Re: [PATCH 1/3] mm: munlock: fix a bug where THP tail page is encountered"
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