RE: Alloc and lock down large amounts of memory

From: Mike Galbraith (
Date: Tue Aug 20 2002 - 23:43:46 EST

At 04:47 PM 8/20/2002 -0400, Richard B. Johnson wrote:
>On Tue, 20 Aug 2002, Bhavana Nagendra wrote:
> > >
> > > Curiosity: why do you want to do device DMA buffer
> > > allocation from userland?
> >
> > I need 256M memory for a graphics operation. It's a requiremment,
> > can't change it. There will be other reasonably sized allocs in kernel
> > space, this is a special case that will be done from userland. As
> > discussed earlier in this thread, there's no good way of alloc()ing
> > and pinning that much in DMA memory space, is there?
> >
> > Gilad, I looked at mm/memory.c and map_user_kiobuf() lets me
> > map user memory into kernel memory and pins it down. A scatter
> > gatter mapping (say, pci_map_sg()) will create a seemingly
> > contiguous buffer for DMA purposes. Does that sound right to you?
> >
> > Bhavana
>You have to cheat. You can tell the kernel that you only have, say
>128 Meg of RAM.

Why not just use early allocation? (if he has eg a 16G box, chopping
it down enough to get at 256M of DMA ram just ain't gonna work:)


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