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

Nicholas Piggin
2020-12-05T04:50:04Z

Excerpts from Edgecombe, Rick P's message of December 5, 2020 4:33 am:
> 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 It's very out of date now but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.

Yeah good question, VM_HUGE might be safer but maybe it would be
possible there's only a few problems that have to be annotated with
VM_NOHUGE, good point. I'll dig it out and see.

> 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/

Oh nice that's what I wanted to do next! We should try get this work
for x86 as well then.

Thanks,
Nick