linux-kernel - Re: [PATCH v4 07/15] mm: pgtable: introduce pagetable

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Message-ID: <e1de887c-6193-48ee-a9b3-04c8a0cdda45@bytedance.com>
Date: Mon, 6 Jan 2025 18:55:58 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: peterz@...radead.org, kevin.brodsky@....com, palmer@...belt.com,
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Subject: Re: [PATCH v4 07/15] mm: pgtable: introduce pagetable_dtor()



On 2025/1/6 18:34, Alexander Gordeev wrote:
> On Mon, Dec 30, 2024 at 05:07:42PM +0800, Qi Zheng wrote:
>> The pagetable_p*_dtor() are exactly the same except for the handling of
>> ptlock. If we make ptlock_free() handle the case where ptdesc->ptl is
>> NULL and remove VM_BUG_ON_PAGE() from pmd_ptlock_free(), we can unify
>> pagetable_p*_dtor() into one function. Let's introduce pagetable_dtor()
>> to do this.
>>
>> Later, pagetable_dtor() will be moved to tlb_remove_ptdesc(), so that
>> ptlock and page table pages can be freed together (regardless of whether
>> RCU is used). This prevents the use-after-free problem where the ptlock
>> is freed immediately but the page table pages is freed later via RCU.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> Originally-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ...
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 5d82f42ddd5cc..cad11fa10c192 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2992,6 +2992,15 @@ static inline bool ptlock_init(struct ptdesc *ptdesc) { return true; }
>>   static inline void ptlock_free(struct ptdesc *ptdesc) {}
>>   #endif /* defined(CONFIG_SPLIT_PTE_PTLOCKS) */
>>   
>> +static inline void pagetable_dtor(struct ptdesc *ptdesc)
>> +{
>> +	struct folio *folio = ptdesc_folio(ptdesc);
>> +
>> +	ptlock_free(ptdesc);
>> +	__folio_clear_pgtable(folio);
>> +	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
>> +}
>> +
> 
> If I am not mistaken, it is just pagetable_pte_dtor() rename.
> What is the point in moving the code around?

No, this is to unify pagetable_p*_dtor() into pagetable_dtor(), so
that we can move pagetable_dtor() to __tlb_remove_table(), and then
ptlock and PTE page can be freed together through RCU, which is
also the main purpose of this patch series.

Thanks!

> 
>>   static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
>>   {
>>   	struct folio *folio = ptdesc_folio(ptdesc);
>> @@ -3003,15 +3012,6 @@ static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
>>   	return true;
>>   }
>>   
>> -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);
>> -}
>> -
>>   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,
>>   			pmd_t *pmdvalp)