Re: [PATCH] arm64, mm: avoid always making PTE dirty in pte_mkwrite()

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Hi, Catalin,

Catalin Marinas <catalin.marinas@arm.com> writes:

> On Wed, Oct 15, 2025 at 10:37:12AM +0800, Huang Ying wrote:
>> Current pte_mkwrite_novma() makes PTE dirty unconditionally.  This may
>> mark some pages that are never written dirty wrongly.  For example,
>> do_swap_page() may map the exclusive pages with writable and clean PTEs
>> if the VMA is writable and the page fault is for read access.
>> However, current pte_mkwrite_novma() implementation always dirties the
>> PTE.  This may cause unnecessary disk writing if the pages are
>> never written before being reclaimed.
>> 
>> So, change pte_mkwrite_novma() to clear the PTE_RDONLY bit only if the
>> PTE_DIRTY bit is set to make it possible to make the PTE writable and
>> clean.
>> 
>> The current behavior was introduced in commit 73e86cb03cf2 ("arm64:
>> Move PTE_RDONLY bit handling out of set_pte_at()").  Before that,
>> pte_mkwrite() only sets the PTE_WRITE bit, while set_pte_at() only
>> clears the PTE_RDONLY bit if both the PTE_WRITE and the PTE_DIRTY bits
>> are set.
>> 
>> To test the performance impact of the patch, on an arm64 server
>> machine, run 16 redis-server processes on socket 1 and 16
>> memtier_benchmark processes on socket 0 with mostly get
>> transactions (that is, redis-server will mostly read memory only).
>> The memory footprint of redis-server is larger than the available
>> memory, so swap out/in will be triggered.  Test results show that the
>> patch can avoid most swapping out because the pages are mostly clean.
>> And the benchmark throughput improves ~23.9% in the test.
>> 
>> Fixes: 73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()")
>> Signed-off-by: Huang Ying <ying.huang@linux.alibaba.com>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Gavin Shan <gshan@redhat.com>
>> Cc: Ard Biesheuvel <ardb@kernel.org>
>> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>> Cc: Yicong Yang <yangyicong@hisilicon.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> ---
>>  arch/arm64/include/asm/pgtable.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index aa89c2e67ebc..0944e296dd4a 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -293,7 +293,8 @@ static inline pmd_t set_pmd_bit(pmd_t pmd, pgprot_t prot)
>>  static inline pte_t pte_mkwrite_novma(pte_t pte)
>>  {
>>  	pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
>> -	pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>> +	if (pte_sw_dirty(pte))
>> +		pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>>  	return pte;
>>  }
>
> This seems to be the right thing. I recall years ago I grep'ed
> (obviously not hard enough) and most pte_mkwrite() places had a
> pte_mkdirty(). But I missed do_swap_page() and possibly others.

The do_swap_page() change was introduced in June 2024, quite recently.

> For this patch:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks!

> I wonder whether we should also add (as a separate patch):
>
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 830107b6dd08..df1c552ef11c 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -101,6 +101,7 @@ static void __init pte_basic_tests(struct pgtable_debug_args *args, int idx)
>  	WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte))));
>  	WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte, args->vma))));
>  	WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte))));
> +	WARN_ON(pte_dirty(pte_mkwrite_novma(pte_mkclean(pte))));
>  	WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte))));
>  }
>
> For completeness, also (and maybe other combinations):
>
> 	WARN_ON(!pte_write(pte_mkdirty(pte_mkwrite_novma(pte))));

Sure.  Will add another patch for this.

> I cc'ed linux-mm in case we missed anything. If nothing raised, I'll
> queue it next week.

---
Best Regards,
Huang, Ying