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

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Message-ID: <d2877287-d456-44d2-b8ee-be1f27b35a9b@os.amperecomputing.com>
Date: Thu, 4 Sep 2025 07:58:24 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Ryan Roberts <ryan.roberts@....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 9/4/25 4:10 AM, Ryan Roberts wrote:
> 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.

Understood.

Thanks,
Yang

>
>> 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;