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

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Message-ID: <377d74d6-a7f9-4e94-9276-168a26d49210@redhat.com>
Date: Fri, 11 Jul 2025 13:47:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, x86@...nel.org
Cc: 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 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" ?

(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.



-- 
Cheers,

David / dhildenb