linux-kernel - Re: [for-4.4-rc6 PATCH] scatterlist: fix sg

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Date:	Tue, 15 Dec 2015 11:35:24 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	axboe@...com, Russell King <linux@....linux.org.uk>,
	Vitaly Lavrov <vel21ripn@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Christoph Hellwig <hch@....de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [for-4.4-rc6 PATCH] scatterlist: fix sg_phys() masking

On Mon, Dec 14, 2015 at 03:17:39PM -0800, Dan Williams wrote:
> commit db0fa0cb0157 "scatterlist: use sg_phys()" did replacements of the
> form:
> 
>     phys_addr_t phys = page_to_phys(sg_page(s));
>     phys_addr_t phys = sg_phys(s) & PAGE_MASK;
> 
> However, this breaks platforms where sizeof(phys_addr_t) >
> sizeof(unsigned long).  Since PAGE_MASK is an unsigned long this
> inadvertently masks the high bits returned by sg_phys().  Convert to
> PHYSICAL_PAGE_MASK in these cases which will do the proper sign
> extension.
> 
> As caught by the kbuild robot, a generic fallback definition of
> PHYSICAL_PAGE_MASK is needed for several archs.

It is getting late in the cycle already, isn't it better to revert
db0fa0cb0157 for now and queue a fixed version for the next round? Then
you can also replace all the 'sg_phys() & MASK' parts by a new sg helper
which just returns the page-aligned physical address.
This would not only be cleaner than your original patch but also leaves
just one place where you need to fix things if necessary.


	Joerg

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