Re: [PATCH 34/49] gma500: the GEM and GTT code is deviceindependant

[Hugh Dickins]

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
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