RE: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems

From: David Laight <David.Laight@ACULAB.COM>
To: 'Matthew Wilcox' <willy@infradead.org>,
	Jesper Dangaard Brouer <brouer@redhat.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
	Ilias Apalodimas <ilias.apalodimas@linaro.org>,
	Matteo Croce <mcroce@linux.microsoft.com>,
	Grygorii Strashko <grygorii.strashko@ti.com>,
	Arnd Bergmann <arnd@kernel.org>, "Christoph Hellwig" <hch@lst.de>
Subject: RE: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
Date: Sat, 17 Apr 2021 10:59:09 +0000	[thread overview]
Message-ID: <a9db4abd89624c698e86a527b93efbf6@AcuMS.aculab.com> (raw)
In-Reply-To: <20210416152755.GL2531743@casper.infradead.org>

From: Matthew Wilcox
> Sent: 16 April 2021 16:28
> 
> On Thu, Apr 15, 2021 at 08:08:32PM +0200, Jesper Dangaard Brouer wrote:
> > See below patch.  Where I swap32 the dma address to satisfy
> > page->compound having bit zero cleared. (It is the simplest fix I could
> > come up with).
> 
> I think this is slightly simpler, and as a bonus code that assumes the
> old layout won't compile.

Always a good plan.

...
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
> -	return page->dma_addr;
> +	dma_addr_t ret = page->dma_addr[0];
> +	if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +		ret |= (dma_addr_t)page->dma_addr[1] << 32;
> +	return ret;
> +}

Won't some compiler/option combinations generate an
error for the '<< 32' when dma_addr_t is 32bit?

You might need to use a (u64) cast.

	David

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