linux-kernel - Re: [PATCH 34/49] gma500: the GEM and GTT code is device independant

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Date:	Sun, 9 Oct 2011 22:15:00 +0200
From:	Patrik Jakobsson <patrik.r.jakobsson@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>, greg@...ah.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 34/49] gma500: the GEM and GTT code is device independant

On Mon, Jul 11, 2011 at 7:49 PM, Hugh Dickins wrote:
> On Mon, 11 Jul 2011, Alan Cox wrote:
>> > Your <4GB pages won't get swapped out while they're pinned.  But can
>> > it happen that they'd be unpinned, swapped out, swapped back in >4GB
>> > pages, then cause trouble for you when needed again?
>>
>> It does look that way, in which case that will eventually need fixing. At
>> the moment you can't put enough memory into a device using these chips
>> but that won't always be true I imagine.
>
> Thanks, I won't worry about it at this moment, but we'd better not forget.
>
> If it's easy for you to include a WARN_ON_ONCE check (perhaps
> on page_to_pfn(page)), that may be worth doing to remind us.
>
> It's a bit sad to learn this requirement just after I'd completed
> removing the readpage copying code, and a bit strange to have shmem
> confined by hardware constraints; but I guess that's what we took on
> when we opened it up to GEM.
>
> It will probably make sense for me to add synchronous migration when
> a shmem swap page is found not to match the contraints wanted by the
> mapping it goes into: mainly for NUMA, but covering your case too.
>
> Hugh

I think we need to revisit this problem. On 3.1-rc4 with some of my own changes
I've just triggered read_cache_page_gfp in psb_gtt_attach_pages when trying to
set a resolution that doesn't fit in stolen memory. Replacing it with
shmem_read_mapping_page seems to work but how do we go about solving the >4GB
issue? Is it ok for now to just use shmem_read_mapping_page or did any of you
have a better solution?

Thanks
Patrik
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