linux-kernel - Re: [PATCH] lib/raid6: Replace custom zero page with ZERO

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Message-Id: <20250315210631.d79637b9c55c7ee3287aa426@linux-foundation.org>
Date: Sat, 15 Mar 2025 21:06:31 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Song Liu
 <song@...nel.org>, Yu Kuai <yukuai3@...wei.com>, linux-raid@...r.kernel.org
Subject: Re: [PATCH] lib/raid6: Replace custom zero page with ZERO_PAGE

On Sat, 15 Mar 2025 16:06:22 +0800 Herbert Xu <herbert@...dor.apana.org.au> wrote:

> Use the system-wide zero page instead of a custom zero page.
> 

I'll assume the MD maintainers will process this.

> --- a/include/linux/raid/pq.h
> +++ b/include/linux/raid/pq.h
> @@ -11,8 +11,9 @@
>  #ifdef __KERNEL__
>  
>  #include <linux/blkdev.h>
> +#include <linux/mm.h>
>  
> -extern const char raid6_empty_zero_page[PAGE_SIZE];
> +#define raid6_empty_zero_page ((const char *)page_address(ZERO_PAGE(0)))

This will of course meet the aligned(256) requirement. 

I do think it would be nicer to write this as a real inlined C function
and to convert usage sites to raid6_empty_zero_page().  IOW, let's tell
the truth rather than pretending that raid6_empty_zero_page is a global
variable.

>  #else /* ! __KERNEL__ */
>  /* Used for testing in user space */
> diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
> index cd2e88ee1f14..03f1a8c179f7 100644
> --- a/lib/raid6/algos.c
> +++ b/lib/raid6/algos.c
> @@ -18,9 +18,6 @@
>  #else
>  #include <linux/module.h>
>  #include <linux/gfp.h>
> -/* In .bss so it's zeroed */
> -const char raid6_empty_zero_page[PAGE_SIZE] __attribute__((aligned(256)));
> -EXPORT_SYMBOL(raid6_empty_zero_page);
>  #endif

Is there any possibility that the MD drivers will point DMA hardware at
the global zero page, thereby invalidating that page from CPU caches in
some manner?