Re: FW: [PATCH] powerpc/mm: Export HPAGE_SHIFT

On Wed, 04 Feb 2009 12:50:48 +1100 Benjamin Herrenschmidt [off-list ref] wrote:

On Tue, 2009-02-03 at 17:08 -0800, Roland Dreier wrote:

quoted

Forwarding Eli's patch below, since PowerPC guys may have missed it.  I
guess the question for Ben et al is whether there is any issue with
exporting HPAGE_SHIFT for modules (can be EXPORT_SYMBOL_GPL if you feel
it's an internal detail).  It would probably make sense to roll this
change into the mlx4 change that Eli alludes to below and merge through
my tree (with ppc maintainer acks of course), rather than splitting this
patch out and introducing cross-tree dependencies (and also separating
the rationale for the change from the change itself).

Thanks,
  Roland


Drivers may want to take advantage of the large pages used for memory obtained
from hugetlbfs. One example is mlx4_ib which can use much less MTT entries (in
the order of HPAGE_SIZE / PAGE_SIZE) when registering such memory, thus scale
significantly better when registering larger memory regions. Other drivers
could also benefit from this.

Except that we support multiple large page sizes nowadays ... I think
the size can be specified per mountpoint of hugetlbfs no ? Thus things
like mellanox would have to query the page size used for a given
mapping.

Do the generic hugetlbfs code provides such an API ? If not, we may need
to add one.

I think it's something like

	huge_page_size(page_hstate(page))