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

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Message-ID: <alpine.LSU.2.00.1107111033440.1752@sister.anvils>
Date:	Mon, 11 Jul 2011 10:49:31 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	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, 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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