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

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Message-ID: <0dd74f57-902a-4a6d-9f77-31963b5953d1@arm.com>
Date: Thu, 13 Feb 2025 12:02:25 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....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 2/7/25 16:50, Ryan Roberts wrote:
> 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?

The suggestion was to add a small comment in the above helper function explaining
the rationale for various conditions in there while returning either PAGE_SIZE or
CONT_PTE_SIZE to improve readability etc.

> 
>>
>>> +}
>>> +
>>> +#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?

Nothing came up while following this code, it was more of a general observation.

> 
>>
>>> +}
>>> +
>>> +#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!
> 
>