linux-kernel - Re: [PATCH v2 2/2] arm64: mm: implement vmemmap_check

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Message-ID: <39a85800-47c5-4529-906d-5a40e58ce136@arm.com>
Date: Fri, 27 Dec 2024 08:27:18 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Catalin Marinas <catalin.marinas@....com>,
 Zhenhua Huang <quic_zhenhuah@...cinc.com>
Cc: will@...nel.org, ardb@...nel.org, ryan.roberts@....com,
 mark.rutland@....com, joey.gouly@....com, dave.hansen@...ux.intel.com,
 akpm@...ux-foundation.org, chenfeiyang@...ngson.cn, chenhuacai@...nel.org,
 linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] arm64: mm: implement vmemmap_check_pmd for arm64

On 12/21/24 00:05, Catalin Marinas wrote:
> On Mon, Dec 09, 2024 at 05:42:27PM +0800, Zhenhua Huang wrote:
>> vmemmap_check_pmd() is used to determine if needs to populate to base
>> pages. Implement it for arm64 arch.
>>
>> Fixes: 2045a3b8911b ("mm/sparse-vmemmap: generalise vmemmap_populate_hugepages()")
>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.com>
>> ---
>>  arch/arm64/mm/mmu.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> 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.