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

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Message-ID: <Z9Zm9nPjU9kT1lt5@gondor.apana.org.au>
Date: Sun, 16 Mar 2025 13:51:50 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Andrew Morton <akpm@...ux-foundation.org>
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, 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@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt