Re: [mm-unstable PATCH v4 3/9] mm/hugetlb: make pud_huge() and follow_huge_pud() aware of non-present pud entry

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/5 17:04, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jul 05, 2022 at 10:46:09AM +0800, Miaohe Lin wrote:
>> On 2022/7/4 9:33, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>>
>>> follow_pud_mask() does not support non-present pud entry now.  As long as
>>> I tested on x86_64 server, follow_pud_mask() still simply returns
>>> no_page_table() for non-present_pud_entry() due to pud_bad(), so no severe
>>> user-visible effect should happen.  But generally we should call
>>> follow_huge_pud() for non-present pud entry for 1GB hugetlb page.
>>>
>>> Update pud_huge() and follow_huge_pud() to handle non-present pud entries.
>>> The changes are similar to previous works for pud entries commit e66f17ff7177
>>> ("mm/hugetlb: take page table lock in follow_huge_pmd()") and commit
>>> cbef8478bee5 ("mm/hugetlb: pmd_huge() returns true for non-present hugepage").
>>>
>>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>> ---
>>> v2 -> v3:
>>> - fixed typos in subject and description,
>>> - added comment on pud_huge(),
>>> - added comment about fallback for hwpoisoned entry,
>>> - updated initial check about FOLL_{PIN,GET} flags.
>>> ---
>>>  arch/x86/mm/hugetlbpage.c |  8 +++++++-
>>>  mm/hugetlb.c              | 32 ++++++++++++++++++++++++++++++--
>>>  2 files changed, 37 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
>>> index 509408da0da1..6b3033845c6d 100644
>>> --- a/arch/x86/mm/hugetlbpage.c
>>> +++ b/arch/x86/mm/hugetlbpage.c
>>> @@ -30,9 +30,15 @@ int pmd_huge(pmd_t pmd)
>>>  		(pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
>>>  }
>>>  
>>> +/*
>>> + * pud_huge() returns 1 if @pud is hugetlb related entry, that is normal
>>> + * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
>>> + * Otherwise, returns 0.
>>> + */
>>>  int pud_huge(pud_t pud)
>>>  {
>>> -	return !!(pud_val(pud) & _PAGE_PSE);
>>> +	return !pud_none(pud) &&
>>> +		(pud_val(pud) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
>>>  }
>>
>> Question: Is aarch64 supported too? It seems aarch64 version of pud_huge matches
>> the requirement naturally for me.
> 
> I think that if pmd_huge() and pud_huge() return true for non-present
> pmd/pud entries, that's OK.  Otherwise we need update to support the
> new feature.
> 
> In aarch64, the bits in pte/pmd/pud related to {pmd,pud}_present() and
> {pmd,pud}_huge() seem not to overlap with the bit range for swap type
> and swap offset, so maybe that's fine.  But I recommend to test with
> arm64 if you have access to aarch64 servers.

I see. This series is intended to enable 1GB hugepage support on x86. And if
someone wants to use it in other arches, it's better to have a test first. ;)

Thanks.

> 
>>
>> Anyway, this patch looks good to me.
>>
>> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> 
> Thank you for reviewing.
> 
> - Naoya Horiguchi
>