Re: [PATCH] mm: fix unevictable page reclaim when calling madvise_pageout

[David Hildenbrand <david@redhat.com>]

On 28.10.19 16:45, zhong jiang wrote:
> On 2019/10/28 23:27, David Hildenbrand wrote:
>> On 28.10.19 16:08, zhong jiang wrote:
>>> Recently, I hit the following issue when running in the upstream.
>>>
>>> kernel BUG at mm/vmscan.c:1521!
>>> invalid opcode: 0000 [#1] SMP KASAN PTI
>>> CPU: 0 PID: 23385 Comm: syz-executor.6 Not tainted 5.4.0-rc4+ #1
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
>>> RIP: 0010:shrink_page_list+0x12b6/0x3530 mm/vmscan.c:1521
>>> Code: de f5 ff ff e8 ab 79 eb ff 4c 89 f7 e8 43 33 0d 00 e9 cc f5 ff ff e8 99 79 eb ff 48 c7 c6 a0 34 2b a0 4c 89 f7 e8 1a 4d 05 00 <0f> 0b e8 83 79 eb ff 48 89 d8 48 c1 e8 03 42 80 3c 38 00 0f 85 74
>>> RSP: 0018:ffff88819a3df5a0 EFLAGS: 00010286
>>> RAX: 0000000000040000 RBX: ffffea00061c3980 RCX: ffffffff814fba36
>>> RDX: 00000000000056f7 RSI: ffffc9000c02c000 RDI: ffff8881f70268cc
>>> RBP: ffff88819a3df898 R08: ffffed103ee05de0 R09: ffffed103ee05de0
>>> R10: 0000000000000001 R11: ffffed103ee05ddf R12: ffff88819a3df6f0
>>> R13: ffff88819a3df6f0 R14: ffffea00061c3980 R15: dffffc0000000000
>>> FS:  00007f21b9d8e700(0000) GS:ffff8881f7000000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 0000001b2d621000 CR3: 00000001c8c46004 CR4: 00000000007606f0
>>> DR0: 0000000020000140 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
>>> PKRU: 55555554
>>> Call Trace:
>>>    reclaim_pages+0x499/0x800 mm/vmscan.c:2188
>>>    madvise_cold_or_pageout_pte_range+0x58a/0x710 mm/madvise.c:453
>>>    walk_pmd_range mm/pagewalk.c:53 [inline]
>>>    walk_pud_range mm/pagewalk.c:112 [inline]
>>>    walk_p4d_range mm/pagewalk.c:139 [inline]
>>>    walk_pgd_range mm/pagewalk.c:166 [inline]
>>>    __walk_page_range+0x45a/0xc20 mm/pagewalk.c:261
>>>    walk_page_range+0x179/0x310 mm/pagewalk.c:349
>>>    madvise_pageout_page_range mm/madvise.c:506 [inline]
>>>    madvise_pageout+0x1f0/0x330 mm/madvise.c:542
>>>    madvise_vma mm/madvise.c:931 [inline]
>>>    __do_sys_madvise+0x7d2/0x1600 mm/madvise.c:1113
>>>    do_syscall_64+0x9f/0x4c0 arch/x86/entry/common.c:290
>>>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>
>>> madvise_pageout access the specified range of the vma and isolate
>>> them, then run shrink_page_list to reclaim the memory. But It also
>>> isolate the unevictable page to reclaim. Hence, we can catch the
>>> cases in shrink_page_list.
>>>
>>> We can fix it by preventing unevictable page from isolating.
>>> Another way to fix the issue by removing the condition of
>>> BUG_ON(PageUnevictable(page)) in shrink_page_list. I think it
>>> is better  to use the latter. Because We has taken the unevictable
>>> page and skip it into account in shrink_page_list.
>> I really don't understand the last sentence. Looks like
>> something got messed up :)
> I mean that we will check the page_evictable(page) in shrink_page_list,
> if it is unevictable page, we will put the page back to correct lru.
> 
> Based on the condition, I make the choice. It seems to more simpler.:-)
> 
> Thanks,
> zhong jiang
>>
>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>>> ---
>>>    mm/vmscan.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index f7d1301..1c6e959 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -1524,7 +1524,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>>    		unlock_page(page);
>>>    keep:
>>>    		list_add(&page->lru, &ret_pages);
>>> -		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
>>> +		VM_BUG_ON_PAGE(PageLRU(page), page);
>> So, this comes from
>>
>> commit b291f000393f5a0b679012b39d79fbc85c018233
>> Author: Nick Piggin <npiggin@suse.de>
>> Date:   Sat Oct 18 20:26:44 2008 -0700
>>
>>      mlock: mlocked pages are unevictable
>>      
>>      Make sure that mlocked pages also live on the unevictable LRU, so kswapd
>>      will not scan them over and over again.
>>
>>
>> That patch is fairly old. How come we can suddenly trigger this?
>> Which commit is responsible for that? Was it always broken?
>>
>> I can see that
>>
>> commit ad6b67041a45497261617d7a28b15159b202cb5a
>> Author: Minchan Kim <minchan@kernel.org>
>> Date:   Wed May 3 14:54:13 2017 -0700
>>
>>      mm: remove SWAP_MLOCK in ttu
>>
>> Performed some changes in that area. But also some time ago.
> I think the following patch introduce the issue.
> 
> commit 1a4e58cce84ee88129d5d49c064bd2852b481357
> Author: Minchan Kim <minchan@kernel.org>
> Date:   Wed Sep 25 16:49:15 2019 -0700
> 
>      mm: introduce MADV_PAGEOUT
> 
>      When a process expects no accesses to a certain memory range for a long
> 

CCing Minchan Kim then.

If this is indeed the introducing patch, you probably reference that 
patch in your cover mail somehow. (Fixes: does not apply until upstream)

I am absolutely no expert on vmscan.c, so I'm afraid I can't really 
comment on the details.

-- 

Thanks,

David / dhildenb