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

[王擎 <wangqing@vivo.com>]

>On Tue, Jul 6, 2021 at 10:41 AM 王擎 <wangqing@vivo.com> wrote:
>>
>>
>> >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
>
>Got it. But I don't think the fix you mentioned above was
>appropriate. What if GFP_KERNEL | GFP_ATOMIC?

Yes agree, but I haven't figured out what will happen this way, 
the test has been passed in syzkaller.
Or how about gfp | GFP_ATOMIC & ~GFP_KERNEL ?

Thanks,

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