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From: Matthew Wilcox <willy@infradead.org>
To: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: brouer@redhat.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, netdev@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
	mcroce@linux.microsoft.com, grygorii.strashko@ti.com,
	arnd@kernel.org, hch@lst.de, linux-snps-arc@lists.infradead.org,
	mhocko@kernel.org, mgorman@suse.de
Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
Date: Sat, 17 Apr 2021 21:22:40 +0100
Message-ID: <20210417202240.GS2531743@casper.infradead.org> (raw)
In-Reply-To: <YHspptFx+T588KcG@apalos.home>

On Sat, Apr 17, 2021 at 09:32:06PM +0300, Ilias Apalodimas wrote:
> > +static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> > +{
> > +	page->dma_addr[0] = addr;
> > +	if (sizeof(dma_addr_t) > sizeof(unsigned long))
> > +		page->dma_addr[1] = addr >> 16 >> 16;
> 
> The 'error' that was reported will never trigger right?
> I assume this was compiled with dma_addr_t as 32bits (so it triggered the
> compilation error), but the if check will never allow this codepath to run.
> If so can we add a comment explaining this, since none of us will remember why
> in 6 months from now?

That's right.  I compiled it all three ways -- 32-bit, 64-bit dma, 32-bit long
and 64-bit.  The 32/64 bit case turn into:

	if (0)
		page->dma_addr[1] = addr >> 16 >> 16;

which gets elided.  So the only case that has to work is 64-bit dma and
32-bit long.

I can replace this with upper_32_bits().


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Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-16 23:07 [PATCH 0/2] Change struct page layout for page_pool Matthew Wilcox (Oracle)
2021-04-16 23:07 ` [PATCH 1/2] mm: Fix struct page layout on 32-bit systems Matthew Wilcox (Oracle)
2021-04-17  1:08   ` kernel test robot
2021-04-17  2:45   ` Matthew Wilcox
2021-04-17 18:32     ` Ilias Apalodimas
2021-04-17 20:22       ` Matthew Wilcox [this message]
2021-04-18 11:21         ` Ilias Apalodimas
2021-04-17 21:18     ` David Laight
2021-04-17 22:45       ` Matthew Wilcox
2021-04-20  2:48     ` Vineet Gupta
2021-04-20  3:10       ` Matthew Wilcox
2021-04-20  7:07         ` Arnd Bergmann
2021-04-20 21:14           ` Vineet Gupta
2021-04-20 21:20             ` Arnd Bergmann
2021-04-21  5:50               ` hch
2021-04-21  8:43               ` David Laight
2021-04-21  8:58                 ` Arnd Bergmann
2021-04-20  7:39     ` Geert Uytterhoeven
2021-04-20  8:39       ` David Laight
2021-04-20 11:32       ` Matthew Wilcox
2021-04-17  7:34   ` Jesper Dangaard Brouer
2021-04-20  7:21   ` Rasmus Villemoes
2021-04-16 23:07 ` [PATCH 2/2] mm: Indicate pfmemalloc pages in compound_head Matthew Wilcox (Oracle)
2021-04-17 21:13   ` David Laight
2021-04-17 21:28     ` Matthew Wilcox
2021-04-20  4:03 ` [PATCH 0/2] Change struct page layout for page_pool Michael Ellerman

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