On 7/21/21 4:59 AM, Gavin Shan wrote:
> 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;
> }
But then that does not allocate the PUD element for the test which
subsequently will be skipped. Isn't it ? So if the order is greater
than MAX_ORDER, allocation needs to happen via alloc_contig_pages()
or something similar.
