Date: Fri, 8 Apr 2016 08:56:27 +0200
From: John Einar Reitan <john.reitan@foss.arm.com>
To: Laura Abbott <labbott@redhat.com>
Cc: gregkh@linuxfoundation.org, arve@android.com, devel@driverdev.osuosl.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ion: scatterlist offset not used for buffer map
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On Thu, Apr 07, 2016 at 12:37:50PM -0700, Laura Abbott wrote:
> On 04/07/2016 04:29 AM, John Einar Reitan wrote:
> > ion's default user/kernel page mapping code don't honor the offset
> > option for scatterlists. It uses sg_page and expect the whole page to be
> > mapped, while the offset could dictate an offset within a large page.
> >
> > sg_phys correctly accounts for the offset, so should be used instead.
> >
> 
> Can you be more specific about which heap and which allocation pattern
> is exposing this bug?

The heap that exposed the bug is one I'm developing and will be posting
as a RFC soon. It uses compound pages and an sub-divides it into surface
buffers. The ion buffers are configured to hold sgl's with the compound
page and the correct offset of the buffer, via 
sg_set_page(.., compound_page, .., offset_of_logical_buffer);

sg_phys/sg_virt  includes this offset, but if you poke the sg and extract
the page with sg_page yourself you must include this offset in your
calculations too.