Re: [PATCH v4 10/15] riscv: pgtable: move pagetable_dtor() to __tlb_remove_table()

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2025/1/3 17:13, Qi Zheng wrote:
> 
> 
> On 2025/1/3 16:02, Kevin Brodsky wrote:
>> On 03/01/2025 04:48, Qi Zheng wrote:
>>> Hi Kevin,
>>>
>>> On 2025/1/3 00:53, Kevin Brodsky wrote:
>>>> On 30/12/2024 10:07, Qi Zheng wrote:
>>>>>    static inline void riscv_tlb_remove_ptdesc(struct mmu_gather *tlb,
>>>>> void *pt)
>>>>>    {
>>>>> -    if (riscv_use_sbi_for_rfence())
>>>>> +    if (riscv_use_sbi_for_rfence()) {
>>>>>            tlb_remove_ptdesc(tlb, pt);
>>>>> -    else
>>>>> +    } else {
>>>>> +        pagetable_dtor(pt);
>>>>>            tlb_remove_page_ptdesc(tlb, pt);
>>>>
>>>> I find the imbalance pretty confusing: pagetable_dtor() is called
>>>> explicitly before using tlb_remove_page() but not tlb_remove_ptdesc().
>>>> Doesn't that assume that CONFIG_MMU_GATHER_HAVE_TABLE_FREE is selected?
>>>> Could we not call pagetable_dtor() from 
>>>> __tlb_batch_free_encoded_pages()
>>>> to ensure that the dtor is always called just before freeing, and 
>>>> remove
>>>
>>> In __tlb_batch_free_encoded_pages(), we can indeed detect PageTable()
>>> and call pagetable_dtor() to dtor the page table pages.
>>> But __tlb_batch_free_encoded_pages() is also used to free normal pages
>>> (not page table pages), so I don't want to add overhead there.
>>
>> Interesting, can a tlb batch refer to pages than are not PTPs then?
> 
> Yes, you can see the caller of __tlb_remove_folio_pages() or 
> tlb_remove_page_size().
> 
>>
>>>
>>> But now I think maybe we can do this in tlb_remove_page_ptdesc(), like
>>> this:
>>>
>>> diff --git a/arch/csky/include/asm/pgalloc.h
>>> b/arch/csky/include/asm/pgalloc.h
>>> index f1ce5b7b28f22..e45c7e91dcbf9 100644
>>> --- a/arch/csky/include/asm/pgalloc.h
>>> +++ b/arch/csky/include/asm/pgalloc.h
>>> @@ -63,7 +63,6 @@ static inline pgd_t *pgd_alloc(struct mm_struct *mm)
>>>
>>>   #define __pte_free_tlb(tlb, pte, address)              \
>>>   do {                                                   \
>>> -       pagetable_dtor(page_ptdesc(pte));               \
>>>          tlb_remove_page_ptdesc(tlb, page_ptdesc(pte));  \
>>>   } while (0)
>>>
>>> [...]
>>>
>>> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
>>> index a96d4b440f3da..a59205863f431 100644
>>> --- a/include/asm-generic/tlb.h
>>> +++ b/include/asm-generic/tlb.h
>>> @@ -506,6 +506,7 @@ static inline void tlb_remove_ptdesc(struct
>>> mmu_gather *tlb, void *pt)
>>>   /* Like tlb_remove_ptdesc, but for page-like page directories. */
>>>   static inline void tlb_remove_page_ptdesc(struct mmu_gather *tlb,
>>> struct ptdesc *pt)
>>>   {
>>> +       pagetable_dtor(pt);
>>>          tlb_remove_page(tlb, ptdesc_page(pt));
>>>   }
>>
>> I think this is an interesting idea, it does make arch code easier to
>> follow. OTOH it would have been more natural to me to call
>> pagetable_dtor() from tlb_remove_page(). I can however see that this
>> doesn't work, because tlb_remove_table() is defined as tlb_remove_page()
>> if CONFIG_MMU_GATHER_HAVE_TABLE_FREE isn't selected. Which brings me
>> back to my earlier question: in that case, aren't we missing a call to
>> pagetable_dtor() when using tlb_remove_table() (or tlb_remove_ptdesc())?
> 
> Thank you for pointing this out!
> 
> Now, there are the following architectures selected 
> CONFIG_MMU_GATHER_RCU_TABLE_FREE:
> 
> 1. arm: select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
> 2. arm64: select MMU_GATHER_RCU_TABLE_FREE
> 3. powerpc: select MMU_GATHER_RCU_TABLE_FREE
> 4. riscv: select MMU_GATHER_RCU_TABLE_FREE if SMP && MMU
> 5. s390: select MMU_GATHER_RCU_TABLE_FREE
> 6. sparc: select MMU_GATHER_RCU_TABLE_FREE if SMP
> 7. x86: select MMU_GATHER_RCU_TABLE_FREE    if PARAVIRT
> 
> If CONFIG_MMU_GATHER_TABLE_FREE is selected, an architecture is expected
> to provide __tlb_remove_table(). This patch series modifies the
> __tlb_remove_table() in arm, arm64, riscv, s390 and x86. Among them,
> arm64 and s390 unconditionally select MMU_GATHER_RCU_TABLE_FREE, so we
> only need to double-check arm, riscv and x86.
> 
> For x86, it was called tlb_remove_page() in the non-PARAVIRT case, and I
> added pagetable_dtor() for it explicitly (see patch #11), so this should
> be no problem.
> 
> For riscv, it will only call tlb_remove_ptdesc() in the case of
> SMP && MMU, so this should be no problem.
> 
> For arm, the call to pagetable_dtor() is indeed missed in the
> non-MMU_GATHER_RCU_TABLE_FREE case. This needs to be fixed. But we
> can't fix this by adding pagetable_dtor() to tlb_remove_table(),
> because some architectures call tlb_remove_table() but don't support
> page table statistics, like sparc.
> 
> So a more direct fix might be:
> 
> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> index a59205863f431..0a131444a18ca 100644
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -500,6 +500,9 @@ static inline void tlb_remove_page(struct mmu_gather 
> *tlb, struct page *page)
> 
>   static inline void tlb_remove_ptdesc(struct mmu_gather *tlb, void *pt)
>   {
> +#ifndef CONFIG_MMU_GATHER_TABLE_FREE
> +       pagetable_dtor(pt);
> +#endif
>          tlb_remove_table(tlb, pt);
>   }
> 
> Or fix it directly in arm? Like the following:
> 
> diff --git a/arch/arm/include/asm/tlb.h b/arch/arm/include/asm/tlb.h
> index ea4fbe7b17f6f..cf5d0ca021440 100644
> --- a/arch/arm/include/asm/tlb.h
> +++ b/arch/arm/include/asm/tlb.h
> @@ -43,6 +43,9 @@ __pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte, 
> unsigned long addr)
>          __tlb_adjust_range(tlb, addr - PAGE_SIZE, 2 * PAGE_SIZE);
>   #endif
> 
> +#ifndef CONFIG_MMU_GATHER_TABLE_FREE
> +       pagetable_dtor(ptdesc);
> +#endif
>          tlb_remove_ptdesc(tlb, ptdesc);
>   }

Or can we just not let tlb_remove_table() fall back to
tlb_remove_page()? Like the following:

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index a59205863f431..354ffaa4bd120 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -195,8 +195,6 @@
   *  various ptep_get_and_clear() functions.
   */

-#ifdef CONFIG_MMU_GATHER_TABLE_FREE
-
  struct mmu_table_batch {
  #ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
         struct rcu_head         rcu;
@@ -219,16 +217,6 @@ static inline void __tlb_remove_table(void *table)

  extern void tlb_remove_table(struct mmu_gather *tlb, void *table);

-#else /* !CONFIG_MMU_GATHER_HAVE_TABLE_FREE */
-
-/*
- * Without MMU_GATHER_TABLE_FREE the architecture is assumed to have 
page based
- * page directories and we can use the normal page batching to free them.
- */
-#define tlb_remove_table(tlb, page) tlb_remove_page((tlb), (page))
-
-#endif /* CONFIG_MMU_GATHER_TABLE_FREE */
-
  #ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
  /*
   * This allows an architecture that does not use the linux page-tables for

> 
> Thanks,
> Qi
> 
>>
>> - Kevin