linux-kernel - Re: [PATCH v2 5/6] mm: tlb: Provide flush_*_tlb

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Date:   Mon, 1 Jun 2020 21:36:41 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Catalin Marinas <catalin.marinas@....com>
CC:     <peterz@...radead.org>, <mark.rutland@....com>, <will@...nel.org>,
        <aneesh.kumar@...ux.ibm.com>, <akpm@...ux-foundation.org>,
        <npiggin@...il.com>, <arnd@...db.de>, <rostedt@...dmis.org>,
        <maz@...nel.org>, <suzuki.poulose@....com>, <tglx@...utronix.de>,
        <yuzhao@...gle.com>, <Dave.Martin@....com>, <steven.price@....com>,
        <broonie@...nel.org>, <guohanjun@...wei.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <arm@...nel.org>, <xiexiangyou@...wei.com>,
        <prime.zeng@...ilicon.com>, <zhangshaokun@...ilicon.com>,
        <kuhn.chenqun@...wei.com>
Subject: Re: [PATCH v2 5/6] mm: tlb: Provide flush_*_tlb_range wrappers

On 2020/6/1 19:56, Catalin Marinas wrote:
> Hi Zhenyu,
> 
> On Sat, May 30, 2020 at 06:24:21PM +0800, Zhenyu Ye wrote:
>> On 2020/5/26 22:52, Catalin Marinas wrote:
>>> On Mon, May 25, 2020 at 03:19:42PM +0800, Zhenyu Ye wrote:
>>>> tlb_flush_##_pxx##_range() is used to set tlb->cleared_*,
>>>> flush_##_pxx##_tlb_range() will actually flush the TLB entry.
>>>>
>>>> In arch64, tlb_flush_p?d_range() is defined as:
>>>>
>>>> 	#define flush_pmd_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>>>> 	#define flush_pud_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>>>
>>> Currently, flush_p??_tlb_range() are generic and defined as above. I
>>> think in the generic code they can remain an alias for
>>> flush_tlb_range().
>>>
>>> On arm64, we can redefine them as:
>>>
>>> #define flush_pte_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 3)
>>> #define flush_pmd_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 2)
>>> #define flush_pud_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 1)
>>> #define flush_p4d_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 0)
>>>
>>> (unless the compiler optimises away all the mmu_gather stuff in your
>>> macro above but they don't look trivial to me)
>>
>> I changed generic code before considering that other structures may also
>> use this feature, such as Power9. And Peter may want to replace all
>> flush_tlb_range() by tlb_flush() in the future, see [1] for details.
>>
>> If only enable this feature on aarch64, your codes are better.
>>
>> [1] https://lore.kernel.org/linux-arm-kernel/20200402163849.GM20713@hirez.programming.kicks-ass.net/
> 
> But we change the semantics slightly if we implement these as
> mmu_gather. For example, tlb_end_vma() -> tlb_flush_mmu_tlbonly() ends
> up calling mmu_notifier_invalidate_range() which it didn't before. I
> think we end up invoking the notifier unnecessarily in some cases (see
> the comment in __split_huge_pmd()) or we end up calling the notifier
> twice (e.g. pmdp_huge_clear_flush_notify()).
> 

Yes, so only enable this feature on aarch64 may be better.
I will change this in V4 of this series. [the v3 only has some minor
changes and can be ignored :)]

>>> Also, I don't see the new flush_pte_* and flush_p4d_* macros used
>>> anywhere and I don't think they are needed. The pte equivalent is
>>> flush_tlb_page() (we need to make sure it's not used on a pmd in the
>>> hugetlb context).
>>
>> flush_tlb_page() is used to flush only one page.  If we add the
>> flush_pte_tlb_range(), then we can use it to flush a range of pages in
>> the future.
> 
> If we know flush_tlb_page() is only called on a small page, could we add
> TTL information here as well?
> 

Yes, we could. I will add this in flush_tlb_page().

>> But flush_pte_* and flush_p4d_* macros are really not used anywhere. I
>> will remove them in next version of series, and add them if someone
>> needs.
> 
> I think it makes sense.
> 

Thanks,
Zhenyu