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Message-ID: <e9fe97d4-99d5-443e-b722-43903655a76e@bytedance.com>
Date: Fri, 3 Jan 2025 17:13:12 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: peterz@...radead.org, 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 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);
}
Thanks,
Qi
>
> - Kevin
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