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Message-ID: <31e1a033-00a7-4953-81e7-0caedd0227a9@bytedance.com>
Date: Fri, 3 Jan 2025 17:35:30 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Kevin Brodsky <kevin.brodsky@....com>, peterz@...radead.org
Cc: agordeev@...ux.ibm.com, palmer@...belt.com, tglx@...utronix.de,
david@...hat.com, jannh@...gle.com, hughd@...gle.com, yuzhao@...gle.com,
willy@...radead.org, muchun.song@...ux.dev, vbabka@...nel.org,
lorenzo.stoakes@...cle.com, akpm@...ux-foundation.org, rientjes@...gle.com,
vishal.moola@...il.com, arnd@...db.de, will@...nel.org,
aneesh.kumar@...nel.org, npiggin@...il.com, dave.hansen@...ux.intel.com,
rppt@...nel.org, ryan.roberts@....com, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
linux-arch@...r.kernel.org, linux-csky@...r.kernel.org,
linux-hexagon@...r.kernel.org, loongarch@...ts.linux.dev,
linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
linux-openrisc@...r.kernel.org, linux-sh@...r.kernel.org,
linux-um@...ts.infradead.org
Subject: Re: [PATCH v4 10/15] riscv: pgtable: move pagetable_dtor() to
__tlb_remove_table()
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
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