linux-kernel - Re: [PATCH v7 6/6] arm64: mm: Optimize linear_map_split_to

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Message-ID: <2cf02021-10d2-46f6-b3e6-f5c55546ca9e@arm.com>
Date: Thu, 4 Sep 2025 12:10:56 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Yang Shi <yang@...amperecomputing.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Ard Biesheuvel <ardb@...nel.org>, Dev Jain <dev.jain@....com>,
 scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v7 6/6] arm64: mm: Optimize linear_map_split_to_ptes()

On 29/08/2025 23:27, Yang Shi wrote:
> 
> 
> On 8/29/25 4:52 AM, Ryan Roberts wrote:
>> When splitting kernel leaf mappings, either via
>> split_kernel_leaf_mapping_locked() or linear_map_split_to_ptes(),
>> previously a leaf mapping was always split to the next size down. e.g.
>> pud -> contpmd -> pmd -> contpte -> pte. But for
>> linear_map_split_to_ptes() we can avoid the contpmd and contpte states
>> because we know we want to split all the way down to ptes.
>>
>> This avoids visiting all the ptes in a table if it was created by
>> splitting a pmd, which is noticible on systems with a lot of memory.
> 
> Similar to patch #4, this patch should be squashed into patch #5 IMHO.

That's fine by me. I was just trying to make the review easier by splitting
non-essential stuff out. Let's squash for next version.

> 
> Thanks,
> Yang
> 
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>   arch/arm64/mm/mmu.c | 26 ++++++++++++++++++--------
>>   1 file changed, 18 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 6bd0b065bd97..8e45cd08bf3a 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -550,7 +550,7 @@ static void split_contpte(pte_t *ptep)
>>           __set_pte(ptep, pte_mknoncont(__ptep_get(ptep)));
>>   }
>>   -static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp)
>> +static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>>   {
>>       pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
>>       unsigned long pfn = pmd_pfn(pmd);
>> @@ -568,7 +568,9 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp)
>>           tableprot |= PMD_TABLE_PXN;
>>         prot = __pgprot((pgprot_val(prot) & ~PTE_TYPE_MASK) | PTE_TYPE_PAGE);
>> -    prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>> +    prot = __pgprot(pgprot_val(prot) & ~PTE_CONT);
>> +    if (to_cont)
>> +        prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>>         for (i = 0; i < PTRS_PER_PTE; i++, ptep++, pfn++)
>>           __set_pte(ptep, pfn_pte(pfn, prot));
>> @@ -592,7 +594,7 @@ static void split_contpmd(pmd_t *pmdp)
>>           set_pmd(pmdp, pmd_mknoncont(pmdp_get(pmdp)));
>>   }
>>   -static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp)
>> +static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
>>   {
>>       pudval_t tableprot = PUD_TYPE_TABLE | PUD_TABLE_UXN | PUD_TABLE_AF;
>>       unsigned int step = PMD_SIZE >> PAGE_SHIFT;
>> @@ -611,7 +613,9 @@ static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp)
>>           tableprot |= PUD_TABLE_PXN;
>>         prot = __pgprot((pgprot_val(prot) & ~PMD_TYPE_MASK) | PMD_TYPE_SECT);
>> -    prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>> +    prot = __pgprot(pgprot_val(prot) & ~PTE_CONT);
>> +    if (to_cont)
>> +        prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>>         for (i = 0; i < PTRS_PER_PMD; i++, pmdp++, pfn += step)
>>           set_pmd(pmdp, pfn_pmd(pfn, prot));
>> @@ -669,7 +673,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long
>> addr)
>>       if (!pud_present(pud))
>>           goto out;
>>       if (pud_leaf(pud)) {
>> -        ret = split_pud(pudp, pud, GFP_PGTABLE_KERNEL);
>> +        ret = split_pud(pudp, pud, GFP_PGTABLE_KERNEL, true);
>>           if (ret)
>>               goto out;
>>       }
>> @@ -694,7 +698,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long
>> addr)
>>            */
>>           if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
>>               goto out;
>> -        ret = split_pmd(pmdp, pmd, GFP_PGTABLE_KERNEL);
>> +        ret = split_pmd(pmdp, pmd, GFP_PGTABLE_KERNEL, true);
>>           if (ret)
>>               goto out;
>>       }
>> @@ -771,7 +775,7 @@ static int __init split_to_ptes_pud_entry(pud_t *pudp,
>> unsigned long addr,
>>       int ret = 0;
>>         if (pud_leaf(pud))
>> -        ret = split_pud(pudp, pud, GFP_ATOMIC);
>> +        ret = split_pud(pudp, pud, GFP_ATOMIC, false);
>>         return ret;
>>   }
>> @@ -786,7 +790,13 @@ static int __init split_to_ptes_pmd_entry(pmd_t *pmdp,
>> unsigned long addr,
>>       if (pmd_leaf(pmd)) {
>>           if (pmd_cont(pmd))
>>               split_contpmd(pmdp);
>> -        ret = split_pmd(pmdp, pmd, GFP_ATOMIC);
>> +        ret = split_pmd(pmdp, pmd, GFP_ATOMIC, false);
>> +
>> +        /*
>> +         * We have split the pmd directly to ptes so there is no need to
>> +         * visit each pte to check if they are contpte.
>> +         */
>> +        walk->action = ACTION_CONTINUE;
>>       }
>>         return ret;
>