Re: [PATCH v8 11/12] mm/vmalloc: Hugepage vmalloc mappings

Edgecombe, Rick P
2020-12-04T18:35:21Z

On Fri, 2020-12-04 at 18:12 +1000, Nicholas Piggin wrote:
> Excerpts from Edgecombe, Rick P's message of December 1, 2020 6:21
> am:
> > On Sun, 2020-11-29 at 01:25 +1000, Nicholas Piggin wrote:
> > > Support huge page vmalloc mappings. Config option
> > > HAVE_ARCH_HUGE_VMALLOC
> > > enables support on architectures that define HAVE_ARCH_HUGE_VMAP
> > > and
> > > supports PMD sized vmap mappings.
> > >
> > > vmalloc will attempt to allocate PMD-sized pages if allocating
> > > PMD
> > > size
> > > or larger, and fall back to small pages if that was unsuccessful.
> > >
> > > Allocations that do not use PAGE_KERNEL prot are not permitted to
> > > use
> > > huge pages, because not all callers expect this (e.g., module
> > > allocations vs strict module rwx).
> >
> > Several architectures (x86, arm64, others?) allocate modules
> > initially
> > with PAGE_KERNEL and so I think this test will not exclude module
> > allocations in those cases.
>
> Ah, thanks. I guess archs must additionally ensure that their
> PAGE_KERNEL allocations are suitable for huge page mappings before
> enabling the option.
>
> If there is interest from those archs to support this, I have an
> early (un-posted) patch that adds an explicit VM_HUGE flag that could
> override the pessemistic arch default. It's not much trouble to add
> this
> to the large system hash allocations. It's very out of date now but
> I
> can at least give what I have to anyone doing an arch support that
> wants it.

Ahh, sorry, I totally missed that this was only enabled for powerpc.

That patch might be useful for me actually. Or maybe a VM_NOHUGE, since
there are only a few places where executable vmallocs are created? I'm
not sure what the other issues are.

I am endeavoring to have small module allocations share large pages, so
this infrastructure is a big help already.
https://lore.kernel.org/lkml/20201120202426.18009-1-rick.p.edgecombe@intel.com/

Thanks!