Re:Re: [Phishing Risk] [External] [PATCH] mm: add GFP_ATOMIC flag after local_lock_irqsave

[王擎 <wangqing@vivo.com>]

>On Mon, Jul 5, 2021 at 9:57 PM Wang Qing <wangqing@vivo.com> wrote:
>>
>> Use GFP_ATOMIC when local_lock_irqsave in __alloc_pages_bulk
>>
>> Reported-by: syzbot+e45919db2eab5e837646@syzkaller.appspotmail.com
>> Signed-off-by: Wang Qing <wangqing@vivo.com>
>> ---
>>  mm/page_alloc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d6e94cc..3016ba5
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5309,7 +5309,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>>                 }
>>                 nr_account++;
>>
>> -               prep_new_page(page, 0, gfp, 0);
>> +               prep_new_page(page, 0, gfp | GFP_ATOMIC, 0);
>
>Hi Wang Qing,
>
>I didn't get the point here. IIUC, prep_new_page() will not allocate
>memory. So why do we need GFP_ATOMIC? What I missed here?
>
>Thanks.

prep_new_page() will allocate memory in some scenarios. For details, 
you can check the bugs detected by syzkaller:
https://syzkaller.appspot.com/bug?id=91c2030241ada0e5d21877f8f2f44c98cffc04bb

Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:96
 ___might_sleep.cold+0x1f1/0x237 kernel/sched/core.c:9153
 prepare_alloc_pages+0x3da/0x580 mm/page_alloc.c:5179
 __alloc_pages+0x12f/0x500 mm/page_alloc.c:5375
 alloc_pages+0x18c/0x2a0 mm/mempolicy.c:2272
 stack_depot_save+0x39d/0x4e0 lib/stackdepot.c:303
 save_stack+0x15e/0x1e0 mm/page_owner.c:120
 __set_page_owner+0x50/0x290 mm/page_owner.c:181
 prep_new_page mm/page_alloc.c:2445 [inline]
 __alloc_pages_bulk+0x8b9/0x1870 mm/page_alloc.c:5313

Thanks.

Qing

>
>>                 if (page_list)
>>                         list_add(&page->lru, page_list);
>>                 else
>> --
>> 2.7.4
>>