linux-kernel - Re: [PATCH v2 1/2] tracing: ring-buffer: Have the ring buffer code do the vmap of physical memory

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Message-ID: <Z-4jzImykAbHK2Gz@gondor.apana.org.au>
Date: Thu, 3 Apr 2025 13:59:40 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, mhiramat@...nel.org,
	mark.rutland@....com, mathieu.desnoyers@...icios.com,
	akpm@...ux-foundation.org, vdonnefort@...gle.com, vbabka@...e.cz,
	rppt@...nel.org, kees@...nel.org, tony.luck@...el.com,
	gpiccoli@...lia.com, linux-hardening@...r.kernel.org,
	willy@...radead.org, Eric Biggers <ebiggers@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v2 1/2] tracing: ring-buffer: Have the ring buffer code
 do the vmap of physical memory

Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
> There's a flush_dcache_range() thing too, but I don't see a single use
> of that function in generic code, so I suspect that one is currently
> purely for architecture-specific drivers and doesn't work in egneral.
> 
> So *this* is the kind of "bad in a different way" I could imagine:
> things that probably should be cleaned up and be available to generic
> code, but very few people have cared, and so they are used in ad-hoc
> places and haven't been sufficiently generalized and cleaned up.

There is no fallback implementation of flush_dcache_range for the
architectures that don't need it.

It would be nice to have that, as doing it by hand is pretty silly:

static inline void scatterwalk_done_dst(struct scatter_walk *walk,
					unsigned int nbytes)
{
	scatterwalk_unmap(walk);
	/*
	 * Explicitly check ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE instead of just
	 * relying on flush_dcache_page() being a no-op when not implemented,
	 * since otherwise the BUG_ON in sg_page() does not get optimized out.
	 * This also avoids having to consider whether the loop would get
	 * reliably optimized out or not.
	 */
	if (ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE) {
		struct page *base_page;
		unsigned int offset;
		int start, end, i;

		base_page = sg_page(walk->sg);
		offset = walk->offset;
		start = offset >> PAGE_SHIFT;
		end = start + (nbytes >> PAGE_SHIFT);
		end += (offset_in_page(offset) + offset_in_page(nbytes) +
			PAGE_SIZE - 1) >> PAGE_SHIFT;
		for (i = start; i < end; i++)
			flush_dcache_page(nth_page(base_page, i));
	}

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