Re: [PATCH v2 2/2] arm64: mm: implement vmemmap_check_pmd for arm64

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

On 2025/1/3 2:12, Catalin Marinas wrote:
> On Fri, Dec 27, 2024 at 08:27:18AM +0530, Anshuman Khandual wrote:
>> On 12/21/24 00:05, Catalin Marinas wrote:
>>> On Mon, Dec 09, 2024 at 05:42:27PM +0800, Zhenhua Huang wrote:
>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>>> index fd59ee44960e..41c7978a92be 100644
>>>> --- a/arch/arm64/mm/mmu.c
>>>> +++ b/arch/arm64/mm/mmu.c
>>>> @@ -1169,7 +1169,8 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node,
>>>>   				unsigned long addr, unsigned long next)
>>>>   {
>>>>   	vmemmap_verify((pte_t *)pmdp, node, addr, next);
>>>> -	return 1;
>>>> +
>>>> +	return pmd_sect(*pmdp);
>>
>> Please change this as pmd_sect(READ_ONCE(*pmdp)) instead.
>>
>>>>   }
>>>>   
>>>>   int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>>
>>> Don't we need this patch only if we implement the first one? Please fold
>>> it into the other patch.
>>
>> Seems like these patches might not be related.
>>
>> While creating huge page based vmemmap mapping during vmemmap_populate_hugepages(),
>> vmemmap_check_pmd() validates if a populated (i.e pmd_none) PMD already represents
>> a huge mapping and can be skipped there after.
>>
>> Current implementation for vmemmap_check_pmd() on arm64, unconditionally returns 1
>> thus asserting that the given populated PMD entry is a huge one indeed, which will
>> be the case unless something is wrong. vmemmap_verify() only ensures that the node
>> where the pfn is allocated from is local.
>>
>> int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node,
>>                                  unsigned long addr, unsigned long next)
>> {
>>          vmemmap_verify((pte_t *)pmdp, node, addr, next);
>>          return 1;
>> }
>>
>> However it does not really check the entry to be a section mapping which it should.
>> Returning pmd_sect(READ_ONCE(*pmdp)) is the right thing, which should have been the
>> case from the beginning when vmemmap_check_pmd() was added. I guess because arm64's
>> original vmemmap_populate() checked only for vmemmap_verify() as well. So probably
>> this does not need a "Fixes: " tag.
> 
> I did not say the patch is wrong, only that it wouldn't be needed unless
> we have the other patch in this series. However, if we do apply the
> other patch, we definitely need this change, so keeping them together
> would make it easier to backport.

Hi Catalin,

Based on our current discussion on patchset #1, we will prohibit 
hugepages(vmemmap mapping) for all hotplugging sections...The flow:
vmemmap_populate
	vmemmap_populate_hugepages
		vmemmap_check_pmd

will *only* be called for non-early sections. Therefore, with patchset 
#1, I don't see the patch as essential.. Would it be acceptable if we do 
not backport this patch?  Anshuman's suggestion seems reasonable to me 
and I separated the patch out:
https://lore.kernel.org/lkml/20250102074047.674156-1-quic_zhenhuah@quicinc.com/

Please share your comments and correct me if I'm mistaken :)


>