Re: consistent_alloc() revisited

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

>As far as I can tell there's no problem with get_free_pages() (or
>rather, alloc_pages()), except that we need to add GFP_ATOMIC to the
>flags if we're in interrupt context.
>
>The problems are in get_vm_area() which does a kmalloc() without
>GFP_ATOMIC and in map_page() which can sleep.

(Note do DaveM: this thread from linuxppc-embedded talks about
out consistent_alloc() functions used to get non-cacheable
DMA consistent memory for peripherials on the various non-PCI
type busses you can find on such embedded CPUs)

We can't rely on in_interrupt(). We can be called with a spinlock
held or on the VM path. In those cases, we need GFP_ATOMIC but
in_interrupt() won't be set.

I would rather pass a gfp_mask argument to consistent_alloc. I know
the pci_xxx version lacks that argument, I yet have to talk to DaveM
about it, in the meantime, it should be implemented as calling
consistent_alloc(..., GFP_ATOMIC).

I want the gfp argument to be passed since some drivers will want
a quite large pool to be allocated at insmod/kernel init time,
and I want to let those drivers to be able to sleep while VM
gets the pages back in this case instead of failing as they would
probably do with a GFP_ATOMIC on a machine that have been running
for some time.

Ben.


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