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

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Message-ID: <874iviprkf.fsf@oracle.com>
Date: Fri, 11 Jul 2025 10:26:08 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, 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_*


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.

> (we have a handful of "kernel page" usages, where we talk about non-user space
> allocations)
>
>> + * @page: address of kernel page
>> + *
>> + * Does absolutely no exception handling.
>> + */
>>   static inline void clear_page(void *page)
>>   {
>>   	memset(page, 0, PAGE_SIZE);
>> diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
>> index 015d23f3e01f..28b9adbc5f00 100644
>> --- a/arch/x86/include/asm/page_64.h
>> +++ b/arch/x86/include/asm/page_64.h
>> @@ -40,23 +40,45 @@ extern unsigned long __phys_addr_symbol(unsigned long);
>>     #define __phys_reloc_hide(x)	(x)
>>   -void clear_page_orig(void *page);
>> -void clear_page_rep(void *page);
>> -void clear_page_erms(void *page);
>> +void memzero_page_aligned_unrolled(void *addr, u64 len);
>>   +/*
>> + * clear_page() - clear kernel page.> + * @page: address of kernel page
>
> Same comment as above.


--
ankur