linux-kernel - Re: [PATCH 1/2] mm: pgtable: add missing flag and statistics for kernel PTE page

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Date: Sun, 4 Feb 2024 19:39:38 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: akpm@...ux-foundation.org, arnd@...db.de, muchun.song@...ux.dev,
 david@...hat.com, willy@...radead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: pgtable: add missing flag and statistics for
 kernel PTE page

Hi Mike,

On 2024/2/4 18:58, Mike Rapoport wrote:
> On Thu, Feb 01, 2024 at 04:05:40PM +0800, Qi Zheng wrote:
>> For kernel PTE page, we do not need to allocate and initialize its split
>> ptlock, but as a page table page, it's still necessary to add PG_table
>> flag and NR_PAGETABLE statistics for it.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>>   include/asm-generic/pgalloc.h |  7 ++++++-
>>   include/linux/mm.h            | 21 ++++++++++++++++-----
>>   2 files changed, 22 insertions(+), 6 deletions(-)
> 
> This should also update the architectures that define
> __HAVE_ARCH_PTE_ALLOC_ONE_KERNEL, otherwise NR_PAGETABLE counts will get
> wrong.

Yes, this patchset only focuses on the generic implementation. For those
architectures that define __HAVE_ARCH_PTE_ALLOC_ONE_KERNEL, some reuse
the generic __pte_alloc_one_kernel(), but some have their own customized
implementations, which indeed need to be fixed.

I wasn't familiar with those architectures and didn't investigate why
they couldn't reuse the generic __pte_alloc_one_kernel(), so I didn't
fix them. It would be better if there are maintainers corresponding to
the architecture who can help fix it. After all, they have a better
understanding of the historical background and have a testing
environment. ;)

> 
> Another related thing is that many architectures have custom allocations
> for early page tables and these would also benefit form NR_PAGETABLE
> accounting.

Indeed, this is also a point that can be optimized.

Thanks.

>   
>> diff --git a/include/asm-generic/pgalloc.h b/include/asm-generic/pgalloc.h
>> index 879e5f8aa5e9..908bd9140ac2 100644
>> --- a/include/asm-generic/pgalloc.h
>> +++ b/include/asm-generic/pgalloc.h
>> @@ -23,6 +23,8 @@ static inline pte_t *__pte_alloc_one_kernel(struct mm_struct *mm)
>>   
>>   	if (!ptdesc)
>>   		return NULL;
>> +
>> +	__pagetable_pte_ctor(ptdesc);
>>   	return ptdesc_address(ptdesc);
>>   }
>>   
>> @@ -46,7 +48,10 @@ static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
>>    */
>>   static inline void pte_free_kernel(struct mm_struct *mm, pte_t *pte)
>>   {
>> -	pagetable_free(virt_to_ptdesc(pte));
>> +	struct ptdesc *ptdesc = virt_to_ptdesc(pte);
>> +
>> +	__pagetable_pte_dtor(ptdesc);
>> +	pagetable_free(ptdesc);
>>   }
>>   
>>   /**
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index e442fd0efdd9..e37db032764e 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2922,26 +2922,37 @@ static inline bool ptlock_init(struct ptdesc *ptdesc) { return true; }
>>   static inline void ptlock_free(struct ptdesc *ptdesc) {}
>>   #endif /* USE_SPLIT_PTE_PTLOCKS */
>>   
>> -static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
>> +static inline void __pagetable_pte_ctor(struct ptdesc *ptdesc)
>>   {
>>   	struct folio *folio = ptdesc_folio(ptdesc);
>>   
>> -	if (!ptlock_init(ptdesc))
>> -		return false;
>>   	__folio_set_pgtable(folio);
>>   	lruvec_stat_add_folio(folio, NR_PAGETABLE);
>> +}
>> +
>> +static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
>> +{
>> +	if (!ptlock_init(ptdesc))
>> +		return false;
>> +
>> +	__pagetable_pte_ctor(ptdesc);
>>   	return true;
>>   }
>>   
>> -static inline void pagetable_pte_dtor(struct ptdesc *ptdesc)
>> +static inline void __pagetable_pte_dtor(struct ptdesc *ptdesc)
>>   {
>>   	struct folio *folio = ptdesc_folio(ptdesc);
>>   
>> -	ptlock_free(ptdesc);
>>   	__folio_clear_pgtable(folio);
>>   	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
>>   }
>>   
>> +static inline void pagetable_pte_dtor(struct ptdesc *ptdesc)
>> +{
>> +	ptlock_free(ptdesc);
>> +	__pagetable_pte_dtor(ptdesc);
>> +}
>> +
>>   pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp);
>>   static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr)
>>   {
>> -- 
>> 2.30.2
>>
>>
>