linux-kernel - Re: [PATCH v1 12/16] arm64/mm: Support huge pte-mapped pages in vmap

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Message-ID: <21da59a8-165d-4423-a00d-d5859f42ec11@arm.com>
Date: Fri, 7 Feb 2025 11:20:34 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Anshuman Khandual <anshuman.khandual@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Dev Jain <dev.jain@....com>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Steve Capper <steve.capper@...aro.org>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 12/16] arm64/mm: Support huge pte-mapped pages in vmap

On 07/02/2025 10:04, Anshuman Khandual wrote:
> 
> 
> On 2/5/25 20:39, Ryan Roberts wrote:
>> Implement the required arch functions to enable use of contpte in the
>> vmap when VM_ALLOW_HUGE_VMAP is specified. This speeds up vmap
>> operations due to only having to issue a DSB and ISB per contpte block
>> instead of per pte. But it also means that the TLB pressure reduces due
>> to only needing a single TLB entry for the whole contpte block.
> 
> Right.
> 
>>
>> Since vmap uses set_huge_pte_at() to set the contpte, that API is now
>> used for kernel mappings for the first time. Although in the vmap case
>> we never expect it to be called to modify a valid mapping so
>> clear_flush() should never be called, it's still wise to make it robust
>> for the kernel case, so amend the tlb flush function if the mm is for
>> kernel space.
> 
> Makes sense.
> 
>>
>> Tested with vmalloc performance selftests:
>>
>>   # kself/mm/test_vmalloc.sh \
>> 	run_test_mask=1
>> 	test_repeat_count=5
>> 	nr_pages=256
>> 	test_loop_count=100000
>> 	use_huge=1
>>
>> Duration reduced from 1274243 usec to 1083553 usec on Apple M2 for 15%
>> reduction in time taken.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>  arch/arm64/include/asm/vmalloc.h | 40 ++++++++++++++++++++++++++++++++
>>  arch/arm64/mm/hugetlbpage.c      |  5 +++-
>>  2 files changed, 44 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h
>> index 38fafffe699f..fbdeb40f3857 100644
>> --- a/arch/arm64/include/asm/vmalloc.h
>> +++ b/arch/arm64/include/asm/vmalloc.h
>> @@ -23,6 +23,46 @@ static inline bool arch_vmap_pmd_supported(pgprot_t prot)
>>  	return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS);
>>  }
>>  
>> +#define arch_vmap_pte_range_map_size arch_vmap_pte_range_map_size
>> +static inline unsigned long arch_vmap_pte_range_map_size(unsigned long addr,
>> +						unsigned long end, u64 pfn,
>> +						unsigned int max_page_shift)
>> +{
>> +	if (max_page_shift < CONT_PTE_SHIFT)
>> +		return PAGE_SIZE;
>> +
>> +	if (end - addr < CONT_PTE_SIZE)
>> +		return PAGE_SIZE;
>> +
>> +	if (!IS_ALIGNED(addr, CONT_PTE_SIZE))
>> +		return PAGE_SIZE;
>> +
>> +	if (!IS_ALIGNED(PFN_PHYS(pfn), CONT_PTE_SIZE))
>> +		return PAGE_SIZE;
>> +
>> +	return CONT_PTE_SIZE;
> 
> A small nit:
> 
> Should the rationale behind picking CONT_PTE_SIZE be added here as an in code
> comment or something in the function - just to make things bit clear.

I'm not sure what other size we would pick?

> 
>> +}
>> +
>> +#define arch_vmap_pte_range_unmap_size arch_vmap_pte_range_unmap_size
>> +static inline unsigned long arch_vmap_pte_range_unmap_size(unsigned long addr,
>> +							   pte_t *ptep)
>> +{
>> +	/*
>> +	 * The caller handles alignment so it's sufficient just to check
>> +	 * PTE_CONT.
>> +	 */
>> +	return pte_valid_cont(__ptep_get(ptep)) ? CONT_PTE_SIZE : PAGE_SIZE;
> 
> I guess it is safe to query the CONT_PTE from the mapped entry itself.

Yes I don't see why not. Is there some specific aspect you're concerned about?

> 
>> +}
>> +
>> +#define arch_vmap_pte_supported_shift arch_vmap_pte_supported_shift
>> +static inline int arch_vmap_pte_supported_shift(unsigned long size)
>> +{
>> +	if (size >= CONT_PTE_SIZE)
>> +		return CONT_PTE_SHIFT;
>> +
>> +	return PAGE_SHIFT;
>> +}
>> +
>>  #endif
>>  
>>  #define arch_vmap_pgprot_tagged arch_vmap_pgprot_tagged
>> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
>> index 02afee31444e..a74e43101dad 100644
>> --- a/arch/arm64/mm/hugetlbpage.c
>> +++ b/arch/arm64/mm/hugetlbpage.c
>> @@ -217,7 +217,10 @@ static void clear_flush(struct mm_struct *mm,
>>  	for (i = 0; i < ncontig; i++, addr += pgsize, ptep++)
>>  		___ptep_get_and_clear(mm, ptep, pgsize);
>>  
>> -	__flush_hugetlb_tlb_range(&vma, saddr, addr, pgsize, true);
>> +	if (mm == &init_mm)
>> +		flush_tlb_kernel_range(saddr, addr);
>> +	else
>> +		__flush_hugetlb_tlb_range(&vma, saddr, addr, pgsize, true);
>>  }
>>  
>>  void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
> 
> Otherwise LGTM.
> 
> Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>

Thanks!