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

[zhong jiang <zhongjiang@huawei.com>]

On 2019/10/29 16:11, Michal Hocko wrote:
> [Cc Minchan]
>
> On Mon 28-10-19 23:08:37, 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.
> The justification is indeed not clear. This is essentially the same kind
> of bug as a58f2cef26e1 ("mm/vmscan.c: fix trying to reclaim unevictable
> LRU page") which has been fixed by checking PageUnevictable before
> adding it to the list of pages to reclaim.
According to the above some bugfix from Minchan and keep the long existing BUG_ON.
Maybe It is better to do that.
> Removing a long existing BUG_ON begs for a much better explanation.
> shrink_page_list is not a trivial piece of code but I _suspect_ that
> removing it should be ok for mapped pages at least (try_to_unmap) but I
> am not so sure how unmapped unevictable pages are handled from top of my
> head.
As to the unmapped unevictable pages.  shrink_page_list has taken that into account.

shinkr_page_list
     page_evictable     --> will filter the unevictable pages to putback its lru.

That should be not a problem after removing the BUG_ON.
> Please also ad Fixes: $sha
Will add in v2.  Thanks
>> 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);
>>  	}
>>  
>>  	pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
>> -- 
>> 1.7.12.4
>>