linux-kernel - Re: [PATCH v5 10/14] x86/mm: Simplify clear_page

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Message-ID: <4253b58b-374d-40b4-87d8-f66926d65676@redhat.com>
Date: Fri, 11 Jul 2025 21:27:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, mingo@...hat.com, mjguzik@...il.com, luto@...nel.org,
 peterz@...radead.org, acme@...nel.org, namhyung@...nel.org,
 tglx@...utronix.de, willy@...radead.org, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v5 10/14] x86/mm: Simplify clear_page_*

On 11.07.25 21:24, Ankur Arora wrote:
> 
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 11.07.25 19:26, Ankur Arora wrote:
>>> David Hildenbrand <david@...hat.com> writes:
>>>
>>>> On 10.07.25 02:59, Ankur Arora wrote:
>>>>> clear_page_rep() and clear_page_erms() are wrappers around "REP; STOS"
>>>>> variations. Inlining gets rid of an unnecessary CALL/RET (which isn't
>>>>> free when using RETHUNK speculative execution mitigations.)
>>>>> Fixup and rename clear_page_orig() to adapt to the changed calling
>>>>> convention.
>>>>> And, add a comment from Dave Hansen detailing various clearing mechanisms
>>>>> used in clear_page().
>>>>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>>>>> ---
>>>>>     arch/x86/include/asm/page_32.h |  6 +++++
>>>>>     arch/x86/include/asm/page_64.h | 42 ++++++++++++++++++++++++++--------
>>>>>     arch/x86/lib/clear_page_64.S   | 39 +++++++------------------------
>>>>>     3 files changed, 46 insertions(+), 41 deletions(-)
>>>>> diff --git a/arch/x86/include/asm/page_32.h b/arch/x86/include/asm/page_32.h
>>>>> index 0c623706cb7e..a8ff43bb9652 100644
>>>>> --- a/arch/x86/include/asm/page_32.h
>>>>> +++ b/arch/x86/include/asm/page_32.h
>>>>> @@ -17,6 +17,12 @@ extern unsigned long __phys_addr(unsigned long);
>>>>>       #include <linux/string.h>
>>>>>     +/*
>>>>
>>>> /** if this was supposed to be kernel doc (which it looks like it is)
>>>>
>>>>> + * clear_page() - clear kernel page.
>>>>
>>>> "clear a kernel page"
>>>>
>>>> Although I am not sure what a "kernel page" is.
>>>>
>>>> Did you mean "clear a page using a kernel virtual address" ?
>>> Thanks. Yes, this makes way more sense.
>>
>> FWIW, most clear_user_page() will just call clear_page(). The ones that don't,
>> have to deal with the dcache flushing, as the page might have cache alias from
>> another (mm) address space.
>>
>> So clear_page() is just about clearing page content using a kernel virtual
>> address, and clear_user_page() is additionally taking care of any dcache
>> aliases.
> 
> Yeah that's a good point.
> 
> Right now I'm assuming that clear_user_page() and clear_page() are
> effectively identical. Which as you say, isn't true for all the
> architectures that can't disambiguate dcache aliases.
> 
> So maybe I should be adjusting the naming of the helpers a bit?
> 
> Specifically, by calling clear_user_pages() from clear_pages_resched().
> Which on x86 etc can just call clear_pages().

Right, and calling it from folio_zero_user() :)

-- 
Cheers,

David / dhildenb