[PATCH] lib/raid6: Replace custom zero page with ZERO_PAGE

[PATCH] lib/raid6: Replace custom zero page with ZERO_PAGE

[Herbert Xu]

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

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/include/linux/raid/pq.h b/include/linux/raid/pq.h
index 98030accf641..1460c15dea63 100644
--- 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)))
 
 #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
 
 struct raid6_calls raid6_call;
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] lib/raid6: Replace custom zero page with ZERO_PAGE

[Andrew Morton]

On Sat, 15 Mar 2025 16:06:22 +0800 Herbert Xu <herbert@gondor.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?

Re: [PATCH] lib/raid6: Replace custom zero page with ZERO_PAGE

[Herbert Xu]

On Sat, Mar 15, 2025 at 09:06:31PM -0700, Andrew Morton wrote:
>
> 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.

OK I can do that.
 
> 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?

The only spots that can hand this off to DMA are:

crypto/async_tx/async_pq.c:                     srcs[i] = (void*)raid6_empty_zero_page;
crypto/async_tx/async_raid6_recov.c:                            ptrs[i] = (void *) raid6_empty_zero_page;
crypto/async_tx/async_raid6_recov.c:                            ptrs[i] = (void*)raid6_empty_zero_page;

But they all turn out to be synchronous fallback code paths that
do not involve DMA at all.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] lib/raid6: Replace custom zero page with ZERO_PAGE

[Andrew Morton]

On Sun, 16 Mar 2025 13:51:50 +0800 Herbert Xu <herbert@gondor.apana.org.au> wrote:

> > 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?
> 
> The only spots that can hand this off to DMA are:

Cool, thanks for checking.

> crypto/async_tx/async_pq.c:                     srcs[i] = (void*)raid6_empty_zero_page;
> crypto/async_tx/async_raid6_recov.c:                            ptrs[i] = (void *) raid6_empty_zero_page;
> crypto/async_tx/async_raid6_recov.c:                            ptrs[i] = (void*)raid6_empty_zero_page;
> 
> But they all turn out to be synchronous fallback code paths that
> do not involve DMA at all.

And all three cast a pointer to a void* ;)