Re: [PATCH v3 3/3] arm64: mm: enable per pmd page table lock

[Anshuman Khandual]

On 03/11/2019 05:42 PM, Mark Rutland wrote:
> Hi,
> 
> On Sat, Mar 09, 2019 at 06:19:06PM -0700, Yu Zhao wrote:
>> Switch from per mm_struct to per pmd page table lock by enabling
>> ARCH_ENABLE_SPLIT_PMD_PTLOCK. This provides better granularity for
>> large system.
>>
>> I'm not sure if there is contention on mm->page_table_lock. Given
>> the option comes at no cost (apart from initializing more spin
>> locks), why not enable it now.
>>
>> We only do so when pmd is not folded, so we don't mistakenly call
>> pgtable_pmd_page_ctor() on pud or p4d in pgd_pgtable_alloc(). (We
>> check shift against PMD_SHIFT, which is same as PUD_SHIFT when pmd
>> is folded).
> 
> Just to check, I take it pgtable_pmd_page_ctor() is now a NOP when the
> PMD is folded, and this last paragraph is stale?
> 
>> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
>> ---
>>  arch/arm64/Kconfig               |  3 +++
>>  arch/arm64/include/asm/pgalloc.h | 12 +++++++++++-
>>  arch/arm64/include/asm/tlb.h     |  5 ++++-
>>  3 files changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index cfbf307d6dc4..a3b1b789f766 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -872,6 +872,9 @@ config ARCH_WANT_HUGE_PMD_SHARE
>>  config ARCH_HAS_CACHE_LINE_SIZE
>>  	def_bool y
>>  
>> +config ARCH_ENABLE_SPLIT_PMD_PTLOCK
>> +	def_bool y if PGTABLE_LEVELS > 2
>> +
>>  config SECCOMP
>>  	bool "Enable seccomp to safely compute untrusted bytecode"
>>  	---help---
>> diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
>> index 52fa47c73bf0..dabba4b2c61f 100644
>> --- a/arch/arm64/include/asm/pgalloc.h
>> +++ b/arch/arm64/include/asm/pgalloc.h
>> @@ -33,12 +33,22 @@
>>  
>>  static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
>>  {
>> -	return (pmd_t *)__get_free_page(PGALLOC_GFP);
>> +	struct page *page;
>> +
>> +	page = alloc_page(PGALLOC_GFP);
>> +	if (!page)
>> +		return NULL;
>> +	if (!pgtable_pmd_page_ctor(page)) {
>> +		__free_page(page);
>> +		return NULL;
>> +	}
>> +	return page_address(page);
>>  }
>>  
>>  static inline void pmd_free(struct mm_struct *mm, pmd_t *pmdp)
>>  {
>>  	BUG_ON((unsigned long)pmdp & (PAGE_SIZE-1));
>> +	pgtable_pmd_page_dtor(virt_to_page(pmdp));
>>  	free_page((unsigned long)pmdp);
>>  }
> 
> It looks like arm64's existing stage-2 code is inconsistent across
> alloc/free, and IIUC this change might turn that into a real problem.
> Currently we allocate all levels of stage-2 table with
> __get_free_page(), but free them with p?d_free(). We always miss the
> ctor and always use the dtor.
> 
> Other than that, this patch looks fine to me, but I'd feel more
> comfortable if we could first fix the stage-2 code to free those stage-2
> tables without invoking the dtor.

Thats right. I have already highlighted this problem.
 
> 
> Anshuman, IIRC you had a patch to fix the stage-2 code to not invoke the
> dtors. If so, could you please post that so that we could take it as a
> preparatory patch for this series?

Sure I can after fixing PTE level pte_free_kernel/__free_page which I had
missed in V2.

https://www.spinics.net/lists/arm-kernel/msg710118.html