Hello,

On Wednesday, August 17, 2011 10:01 AM Marek Szyprowski wrote:

On Tuesday, August 16, 2011 4:26 PM Arnd Bergmann wrote:

quoted

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

quoted

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.

Do we really need the dynamic pool for the first version? I would like to
know how much memory can be allocated in GFP_ATOMIC context. What are the
typical sizes of such allocations?

Maybe for the first version a static pool with reasonably small size
(like 128KiB) will be more than enough? This size can be even board
depended or changed with kernel command line for systems that really
needs more memory.

I noticed one more problem. The size of the CMA managed area must be
the multiple of 16MiBs (MAX_ORDER+1). This means that the smallest CMA area
is 16MiB. These values comes from the internals of the kernel memory
management design and page blocks are the only entities that can be managed
with page migration code.

I'm really sorry for the confusion. This 16MiB value worried me too much and
I've checked the code once again and found that this MAX_ORDER+1 value was
a miscalculation, which appeared in v11 of the  patches. The true minimal
CMA area size is 8MiB for ARM architecture. I believe this shouldn't be
an issue for the current ARMv6+ based machines.

I've checked it with "mem=16M cma=8M" kernel arguments. System booted fine
and CMA area has been successfully created.

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center

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