linux-kernel - Re: [PATCH v2 01/12] mm/debug_vm_pgtable: Introduce struct pgtable_debug

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Date:   Wed, 21 Jul 2021 09:29:43 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        will@...nel.org, akpm@...ux-foundation.org, chuhu@...hat.com,
        shan.gavin@...il.com
Subject: Re: [PATCH v2 01/12] mm/debug_vm_pgtable: Introduce struct
 pgtable_debug_args

On 7/20/21 4:42 PM, Anshuman Khandual wrote:
> On 7/19/21 6:31 PM, Gavin Shan wrote:
>>> +    if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>>> +        has_transparent_hugepage()) {
>>> +        page = alloc_pages(GFP_KERNEL, HPAGE_PMD_ORDER);
>>> +        if (page) {
>>> +            args->pmd_pfn = page_to_pfn(page);
>>> +            args->pte_pfn = args->pmd_pfn;
>>> +            return 0;
>>> +        }
>>> +    }
>>> +
>>
>> As syzbot reported against v1 series, we could allocate pages larger than (1 << (MAX_ORDER - 1)) here.
>> So __GFP_NOWARN is needed here. I will fix it in v3 series.
> 
> I could find the following build error reported from lkp on V2.
> 
> mm/debug_vm_pgtable.c:445:8: warning: variable 'pud' set but not used [-Wunused-but-set-variable]
> 

Yes, The following line is missed in PATCH[v2 09/12] and fixed in
PATCH[v3 09/12]: WARN_ON(!pud_none(pud)). With this line added,
the variable @pud is used in v3.

> Could you please point to the syzbot reported problem on V1 as you
> have mentioned above. Are there configs where HPAGE_[PMD|PUD]_ORDER
> is greater than (MAX_ORDER - 1) ? If yes, how adding __GFP_NOWARN
> solves the problem ?
> 

https://syzkaller.appspot.com/bug?extid=8730ec44a441a434a2c8
https://syzkaller.appspot.com/x/.config?x=29a82c885e192046

The kernel config has the following options:

CONFIG_X86_64=y
CONFIG_TRANSPARENT_HUGEPAGE=y
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD=y
#define PUD_SHIFT        30
#define PMD_SHIFT        21

CONFIG_FORCE_MAX_ZONEORDER=n
#define MAX_ORDER	11

(HPAGE_PUD_SHIFT - PAGE_SHIFT) >= (1 << MAX_ORDER)
(HPAGE_PMD_ORDER)              <  (1 << MAX_ORDER)

The warning is triggered in the following path, __GFP_NOWARN helps to
avoid the WARNING_ON_ONCE(), but NULL is returned as expected.

    alloc_pages
      __alloc_pages

        if (unlikely(order >= MAX_ORDER)) {
                 WARN_ON_ONCE(!(gfp & __GFP_NOWARN));
                 return NULL;
         }

Thanks,
Gavin