Re: [Intel-gfx] [PATCH 02/13] drm/i915: rewrite shmem_pwrite_slow to use copy_from_user

Ben Widawsky
2011-11-21T17:55:07Z

On Mon, Nov 21, 2011 at 05:02:44PM +0100, Daniel Vetter wrote:
> On Sun, Nov 20, 2011 at 09:56:32PM -0800, Ben Widawsky wrote:
> [snip the patch]
> > Bikeshed, but I would much prefer a #define for the swizzle
> > bit/cacheline size.
>
> I've looked at this stuff way too long, so I'm biased, but 64 = cacheline
> = dram fetch size = 1 << 64 feels about as natural for me as 4096 =
> PAGE_SIZE ...
>
> [snip the patch]
>
> > I must be missing something obvious here...
> > Can you explain how this can possibly be considered safe without holding
> > struct_mutex?
>
> That's the reason why the commit msg goes through every case and explains
> why I think it's safe. The large thing here is that we need to drop the
> mutex when calling copy_*_user (at least in the non-atomic slow-paths)
> because otherwise we might deadlock with our own pagefault handler.
> -Daniel

The part about dropping struct_mutex is clear to me.

The bit that I'm missing, I just don't see how you guarantee the page
you're reading from (assuming it's a GTT mmapped page) doesn't get moved
from out under you. For instance if the page isn't there when you do the
initial __copy_from_user, it will get faulted in... cool - but what if
somewhere in that loop the object gets swapped out and something else is
put in it's place? How is that prevented?

Sorry if it's a stupid question, I just don't get it.
Ben