Re: [PATCH v6 01/12] mm/sparsemem: Introduce struct mem_section_usage

From: Pavel Tatashin
Date: Sat May 04 2019 - 11:57:59 EST

> > I'm ok with it being 16M for now unless it causes a problem in
> > practice, i.e. something like the minimum hardware mapping alignment
> > for physical memory being less than 16M.
> On second thought, arbitrary differences across architectures is a bit
> sad. The most common nvdimm namespace alignment granularity is
> PMD_SIZE, so perhaps the default sub-section size should try to match
> that default.

I think that even if you keep it 16M for now, at very least you should
make the map_active bitmap scalable so it will be possible to change
as required later without revisiting all functions that use it. Making
it a static array won't slowdown x86, as it will be still a single
64-bit word on x86.