Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs

[Vlastimil Babka <vbabka@suse.cz>]

On 12/18/2013 05:12 AM, Wanpeng Li wrote:
> Hi Sasha,
> On Tue, Dec 17, 2013 at 10:32:53PM -0500, Sasha Levin wrote:
>> On 12/17/2013 10:23 PM, Wanpeng Li wrote:
>>> -			mlock_vma_page(page);   /* no-op if already mlocked */
>>> -			if (page == check_page)
>>> +			if (page != check_page && trylock_page(page)) {
>>> +				mlock_vma_page(page);   /* no-op if already mlocked */
>>> +				unlock_page(page);
>>> +			} else if (page == check_page) {
>>> +				mlock_vma_page(page);  /* no-op if already mlocked */
>>>   				ret = SWAP_MLOCK;
>>> +			}
>>
>> Previously, if page != check_page and the page was locked, we'd call mlock_vma_page()
>> anyways. With this change, we don't. In fact, we'll just skip that entire block not doing
>> anything.
>
> Thanks for pointing out. ;-)
>
>>
>> If that's something that's never supposed to happen, can we add a
>>
>> 	VM_BUG_ON(page != check_page && PageLocked(page))
>>
>> Just to cover this new code path?
>>
>
> How about this one?
>
>
> 0001-3.patch
>
>
>  From eab57d94c82fb3ab74b607a3ede0f5ce765ff2cc Mon Sep 17 00:00:00 2001
> From: Wanpeng Li <liwanp@linux.vnet.ibm.com>
> Date: Wed, 18 Dec 2013 12:05:59 +0800
> Subject: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs
>
> objrmap doesn't work for nonlinear VMAs because the assumption that offset-into-file
> correlates with offset-into-virtual-addresses does not hold. Hence what
> try_to_unmap_cluster does is a mini "virtual scan" of each nonlinear VMA which maps
> the file to which the target page belongs. If vma locked, mlock the pages in the
> cluster, rather than unmapping them. However, not all pages are guarantee page
> locked instead of the check page. This patch fix the BUG by try to lock !check page
> if them are unlocked.
>
> [  253.869145] kernel BUG at mm/mlock.c:82!
> [  253.869549] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [  253.870098] Dumping ftrace buffer:
> [  253.870098]    (ftrace buffer empty)
> [  253.870098] Modules linked in:
> [  253.870098] CPU: 10 PID: 9162 Comm: trinity-child75 Tainted: G        W    3.13.0-rc4-next-20131216-sasha-00011-g5f105ec-dirty #4137
> [  253.873310] task: ffff8800c98cb000 ti: ffff8804d34e8000 task.ti: ffff8804d34e8000
> [  253.873310] RIP: 0010:[<ffffffff81281f28>]  [<ffffffff81281f28>] mlock_vma_page+0x18/0xc0
> [  253.873310] RSP: 0000:ffff8804d34e99e8  EFLAGS: 00010246
> [  253.873310] RAX: 006fffff8038002c RBX: ffffea00474944c0 RCX: ffff880807636000
> [  253.873310] RDX: ffffea0000000000 RSI: 00007f17a9bca000 RDI: ffffea00474944c0
> [  253.873310] RBP: ffff8804d34e99f8 R08: ffff880807020000 R09: 0000000000000000
> [  253.873310] R10: 0000000000000001 R11: 0000000000002000 R12: 00007f17a9bca000
> [  253.873310] R13: ffffea00474944c0 R14: 00007f17a9be0000 R15: ffff880807020000
> [  253.873310] FS:  00007f17aa31a700(0000) GS:ffff8801c9c00000(0000) knlGS:0000000000000000
> [  253.873310] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [  253.873310] CR2: 00007f17a94fa000 CR3: 00000004d3b02000 CR4: 00000000000006e0
> [  253.873310] DR0: 00007f17a74ca000 DR1: 0000000000000000 DR2: 0000000000000000
> [  253.873310] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
> [  253.873310] Stack:
> [  253.873310]  0000000b3de28067 ffff880b3de28e50 ffff8804d34e9aa8 ffffffff8128bc31
> [  253.873310]  0000000000000301 ffffea0011850220 ffff8809a4039000 ffffea0011850238
> [  253.873310]  ffff8804d34e9aa8 ffff880807636060 0000000000000001 ffff880807636348
> [  253.873310] Call Trace:
> [  253.873310]  [<ffffffff8128bc31>] try_to_unmap_cluster+0x1c1/0x340
> [  253.873310]  [<ffffffff8128c60a>] try_to_unmap_file+0x20a/0x2e0
> [  253.873310]  [<ffffffff8128c7b3>] try_to_unmap+0x73/0x90
> [  253.873310]  [<ffffffff812b526d>] __unmap_and_move+0x18d/0x250
> [  253.873310]  [<ffffffff812b53e9>] unmap_and_move+0xb9/0x180
> [  253.873310]  [<ffffffff812b559b>] migrate_pages+0xeb/0x2f0
> [  253.873310]  [<ffffffff812a0660>] ? queue_pages_pte_range+0x1a0/0x1a0
> [  253.873310]  [<ffffffff812a193c>] migrate_to_node+0x9c/0xc0
> [  253.873310]  [<ffffffff812a30b8>] do_migrate_pages+0x1b8/0x240
> [  253.873310]  [<ffffffff812a3456>] SYSC_migrate_pages+0x316/0x380
> [  253.873310]  [<ffffffff812a31ec>] ? SYSC_migrate_pages+0xac/0x380
> [  253.873310]  [<ffffffff811763c6>] ? vtime_account_user+0x96/0xb0
> [  253.873310]  [<ffffffff812a34ce>] SyS_migrate_pages+0xe/0x10
> [  253.873310]  [<ffffffff843c4990>] tracesys+0xdd/0xe2
> [  253.873310] Code: 0f 1f 00 65 48 ff 04 25 10 25 1d 00 48 83 c4 08
> 5b c9 c3 55 48 89 e5 53 48 83 ec 08 66 66 66 66 90 48 8b 07 48 89 fb
> a8 01 75 10 <0f> 0b 66 0f 1f 44 00 00 eb fe 66 0f 1f 44 00 00 f0 0f ba
> 2f 15
> [  253.873310] RIP  [<ffffffff81281f28>] mlock_vma_page+0x18/0xc0
> [  253.873310]  RSP <ffff8804d34e99e8>
> [  253.904194] ---[ end trace be59c4a7f8edab3f ]---
>
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
> ---
>   mm/rmap.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 55c8b8d..1e24813 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1347,6 +1347,7 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
>   	unsigned long end;
>   	int ret = SWAP_AGAIN;
>   	int locked_vma = 0;
> +	int we_locked = 0;
>
>   	address = (vma->vm_start + cursor) & CLUSTER_MASK;
>   	end = address + CLUSTER_SIZE;
> @@ -1385,9 +1386,15 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
>   		BUG_ON(!page || PageAnon(page));
>
>   		if (locked_vma) {
> -			mlock_vma_page(page);   /* no-op if already mlocked */
> -			if (page == check_page)
> +			if (page != check_page) {
> +				we_locked = trylock_page(page);

If it's not us who has the page already locked, but somebody else, he 
might unlock it at this point and then the BUG_ON in mlock_vma_page() 
will trigger again.

> +				mlock_vma_page(page);   /* no-op if already mlocked */
> +				if (we_locked)
> +					unlock_page(page);
> +			} else if (page == check_page) {
> +				mlock_vma_page(page);  /* no-op if already mlocked */
>   				ret = SWAP_MLOCK;
> +			}
>   			continue;	/* don't unmap */
>   		}
>
>

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