[PATCH] hugetlb: allow to free gigantic pages regardless of the configuration

[Vlastimil Babka]

On 1/17/19 7:39 PM, Alexandre Ghiti wrote:
> From: Alexandre Ghiti <alex at ghiti.fr>
> 
> On systems without CMA or (MEMORY_ISOLATION && COMPACTION) activated but
> that support gigantic pages, boottime reserved gigantic pages can not be
> freed at all. This patchs simply enables the possibility to hand back
> those pages to memory allocator.
> 
> This commit then renames gigantic_page_supported and
> ARCH_HAS_GIGANTIC_PAGE to make them more accurate. Indeed, those values
> being false does not mean that the system cannot use gigantic pages: it
> just means that runtime allocation of gigantic pages is not supported,
> one can still allocate boottime gigantic pages if the architecture supports
> it.
> 
> Signed-off-by: Alexandre Ghiti <alex at ghiti.fr>

I'm fine with the change, but wonder if this can be structured better in a way
which would remove the duplicated "if (MEMORY_ISOLATION && COMPACTION) || CMA"
from all arches, as well as the duplicated
gigantic_page_runtime_allocation_supported()

something like:

- "select ARCH_HAS_GIGANTIC_PAGE" has no conditions, it just says the arch can
support them either at boottime or runtime (but runtime is usable only if other
conditions are met)
- gigantic_page_runtime_allocation_supported() is a function that returns true
if ARCH_HAS_GIGANTIC_PAGE && ((MEMORY_ISOLATION && COMPACTION) || CMA) and
there's a single instance, not per-arch.
- code for freeing gigantic pages can probably still be conditional on
ARCH_HAS_GIGANTIC_PAGE

BTW I wanted also to do something about the "(MEMORY_ISOLATION && COMPACTION) ||
CMA" ugliness itself, i.e. put the common parts behind some new kconfig
(COMPACTION_CORE ?) and expose it better to users, but I can take a stab on that
once the above part is settled.

Vlastimil