Messages in this thread | | | Date | Wed, 1 Jul 2015 19:48:43 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] ARM: keystone: allow FORCE_MAX_ZONEORDER to be configurable on Keystone |
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On Wed, Jul 01, 2015 at 01:53:02PM -0400, Murali Karicheri wrote: > Currently for Keystone devices, user can't change the > value of CONFIG_FORCE_MAX_ZONEORDER option in defconfig. > Users require capability to tune the value of this option on a target > board. So this patch adds this capability
No they shouldn't. If we do permit this, it should not be unrestricted - it's a power-of-2 of the page size, so specifying something like 32768 is insane.
In any case, it's well known that the Linux MM system fragments memory, and the higher order allocations will fail soon after boot - and the larger the order, the greater chance of it failing.
The only case that you want large allocations is for things like DMA, and we have a separate allocator for that called CMA, which is able to grab large chunks of memory, provided it's configured with a large enough zone.
Please check whether CMA can be used _before_ considering using this option. If you need to increase the order, it should be justified, and it should be done on a per SoC basis in a static way, not left to the user to dream up some power-of-2 figure.
Most importantly, please explain in the commit message why CMA doesn't provide you with what you need.
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