On Tuesday 16 August 2011, Russell King - ARM Linux wrote:

On Tue, Aug 16, 2011 at 03:28:48PM +0200, Arnd Bergmann wrote:

quoted

Hmm, I don't remember the point about dynamically sizing the pool for
ARMv6K, but that can well be an oversight on my part.  I do remember the
part about taking that memory pool from the CMA region as you say.

If you're setting aside a pool of pages, then you have to dynamically
size it.  I did mention during our discussion about this.

The problem is that a pool of fixed size is two fold: you need it to be
sufficiently large that it can satisfy all allocations which come along
in atomic context.  Yet, we don't want the pool to be too large because
then it prevents the memory being used for other purposes.

Basically, the total number of pages in the pool can be a fixed size,
but as they are depleted through allocation, they need to be
re-populated from CMA to re-build the reserve for future atomic
allocations.  If the pool becomes larger via frees, then obviously
we need to give pages back.

Ok, thanks for the reminder. I must have completely missed this part
of the discussion.

When I briefly considered this problem, my own conclusion was that
the number of atomic DMA allocations would always be very low
because they tend to be short-lived (e.g. incoming network packets),
so we could ignore this problem and just use a smaller reservation
size. While this seems to be true in general (see "git grep -w -A3 
dma_alloc_coherent | grep ATOMIC"), there is one very significant
case that we cannot ignore, which is pci_alloc_consistent.

This function is still called by hundreds of PCI drivers and always
does dma_alloc_coherent(..., GFP_ATOMIC), even for long-lived
allocations and those that are too large to be ignored.

So at least for the case where we have PCI devices, I agree that
we need to have the dynamic pool.

	Arnd

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