Re: [PATCH] ion: scatterlist offset not used for buffer map

[Colin Cross]

On Thu, Apr 7, 2016 at 11:56 PM, John Einar Reitan
<john.reitan@xxxxxxxxxxxx> wrote:
> 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);

I don't think this is right.  A compound_page still has a page struct
for every page, you should be passing the page struct where your data
starts.  Using an offset > PAGE_SIZE is going to break lots of places,
for example anywhere that uses kmap(sg_page(sg)).

> 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.