Re: [PATCH v2 4/4] mm/vmalloc: Hugepage vmalloc mappings

On Mon, Apr 13, 2020 at 10:53:03PM +1000, Nicholas Piggin wrote:

For platforms that define HAVE_ARCH_HUGE_VMAP and support PMD vmap mappings,
have vmalloc attempt to allocate PMD-sized pages first, before falling back
to small pages. Allocations which use something other than PAGE_KERNEL
protections are not permitted to use huge pages yet, not all callers expect
this (e.g., module allocations vs strict module rwx).

This gives a 6x reduction in dTLB misses for a `git diff` (of linux), from
45600 to 6500 and a 2.2% reduction in cycles on a 2-node POWER9.

This can result in more internal fragmentation and memory overhead for a
given allocation. It can also cause greater NUMA unbalance on hashdist
allocations.

There may be other callers that expect small pages under vmalloc but use
PAGE_KERNEL, I'm not sure if it's feasible to catch them all. An
alternative would be a new function or flag which enables large mappings,
and use that in callers.

Why do we even use vmalloc in this case rather than just doing a huge
page allocation?  What callers are you intersted in?