Re: [PATCH 6/8] drivers: add Contiguous Memory Allocator

[Arnd Bergmann]

On Friday 08 July 2011, Russell King - ARM Linux wrote:
> On Tue, Jul 05, 2011 at 03:58:39PM +0200, Arnd Bergmann wrote:
> 
> > If I'm reading your "ARM: DMA: steal memory for DMA coherent mappings"
> > correctly, the idea is to have a per-platform compile-time amount
> > of memory that is reserved purely for coherent allocations and
> > taking out of the buddy allocator, right?
> 
> Yes, because every time I've looked at taking out memory mappings in
> the first level page tables, it's always been a major issue.
> 
> We have a method where we can remove first level mappings on
> uniprocessor systems in the ioremap code just fine - we use that so
> that systems can setup section and supersection mappings.  They can
> tear them down as well - and we update other tasks L1 page tables
> when they get switched in.
> 
> This, however, doesn't work on SMP, because if you have a DMA allocation
> (which is permitted from IRQ context) you must have some way of removing
> the L1 page table entries from all CPUs TLBs and the page tables currently
> in use and any future page tables which those CPUs may switch to.

Ah, interesting. So there is no tlb flush broadcast operation and it
always goes through IPI?

> So, in a SMP system, there is no safe way to remove L1 page table entries
> from IRQ context.  That means if memory is mapped for the buddy allocators
> using L1 page table entries, then it is fixed for that application on a
> SMP system.

Ok. Can we limit GFP_ATOMIC to memory that doesn't need to be remapped then?
I guess we can assume that there is no regression if we just skip
the dma_alloc_contiguous step in dma_alloc_coherent for any atomic
callers and immediately fall back to the regular allocator.

Unfortunately, this still means we have to keep both methods. I was
hoping that with CMA doing dynamic remapping there would be no need for
keeping a significant number of pages reserved for this.

	Arnd
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