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

[Nicholas Piggin <npiggin@gmail.com>]

Mark Rutland's on June 11, 2019 12:10 am:
> Hi,
> 
> On Mon, Jun 10, 2019 at 02:38:38PM +1000, Nicholas Piggin wrote:
>> For platforms that define HAVE_ARCH_HUGE_VMAP, have vmap allow vmalloc to
>> allocate huge pages and map them
>> 
>> This brings dTLB misses for linux kernel tree `git diff` from 45,000 to
>> 8,000 on a Kaby Lake KVM guest with 8MB dentry hash and mitigations=off
>> (performance is in the noise, under 1% difference, page tables are likely
>> to be well cached for this workload). Similar numbers are seen on POWER9.
> 
> Do you happen to know which vmalloc mappings these get used for in the
> above case? Where do we see vmalloc mappings that large?

Large module vmalloc could be subject to huge mappings.

> I'm worried as to how this would interact with the set_memory_*()
> functions, as on arm64 those can only operate on page-granular mappings.
> Those may need fixing up to handle huge mappings; certainly if the above
> is all for modules.

Good point, that looks like it would break on arm64 at least. I'll
work on it. We may have to make this opt in beyond HUGE_VMAP.

Thanks,
Nick