On Thu, Dec 08, 2016 at 08:30:36AM -0800, Scott Branden wrote: > On 16-12-08 02:00 AM, Catalin Marinas wrote: > >On Wed, Dec 07, 2016 at 11:40:00AM -0800, Scott Branden wrote: > >>Make MAX_PHYSMEM_BITS and SECTIONS_SIZE_BITS configurable by adding > >>config options. > >>Default to current settings currently defined in sparesmem.h. > >>For systems wishing to save memory the config options can be overridden. > >>Example, changing MAX_PHYSMEM_BITS from 48 to 36 at the same time as > >>changing SECTION_SIZE_BITS from 30 to 26 frees 13MB of memory. [...] > > I would rather reduce SECTION_SIZE_BITS permanently where > >feasible, like in this patch: > > > >http://lkml.kernel.org/r/1465821119-3384-1-git-send-email-jszh...@marvell.com > > This patch does not meet my requirements as I need SECTION_SIZE_BITS to be > set to 28 to reduce memory
So with this patch, we reduce it to 27, it should be fine-grained enough for 128MB sections. Alternatively, there were other suggestions here: http://lkml.iu.edu/hypermail/linux/kernel/1604.1/03036.html > and to allow memory hotplug to allocate a 256 MB section. Can memory hotplug not work with 2*128MB sections in this case? > My patch future proofs the tuning of the parameters by allowing > any section size to be made. While MAX_PHYSMEM_BITS makes sense to users in general, SECTION_SIZE_BITS is not always clear to the average user what it means and its min/max boundaries. That's another reason (apart from single/few Image case) why I prefer to not expose it as configuration option. > I could combine the patch you list such that > SECTION_SIZE_BITS defaults to 30 when CONFIG_ARM64_64_PAGES is selected and > 27 otherwise. Should it default to something else for 16K and 4K pages? I haven't done any calculations for 16K yet but we could probably come up with some formula based on PAGE_SHIFT to cover all cases. > In terms of MAX_PHYSMEM_BITS, if our SoCs only use 40 (or less) bits I would > also like the configuration functionality. This allows us to make the > SECTION_SIZE_BITS smaller. So how small do you want SECTION_SIZE_BITS to be? As I said above, 128MB sections should be sufficient in most cases and without the need to reduce MAX_PHYSMEM_BITS. -- Catalin