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

[Muchun Song]

On Tue, Jul 6, 2021 at 10:41 AM 王擎 <wangqing@xxxxxxxx> wrote:
>
>
> >On Mon, Jul 5, 2021 at 9:57 PM Wang Qing <wangqing@xxxxxxxx> wrote:
> >>
> >> Use GFP_ATOMIC when local_lock_irqsave in __alloc_pages_bulk
> >>
> >> Reported-by: syzbot+e45919db2eab5e837646@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> Signed-off-by: Wang Qing <wangqing@xxxxxxxx>
> >> ---
> >>  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?

Thanks.

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