Re: [PATCH v2 01/11] mm/hmm: select mmu notifier when selecting HMM

Jerome Glisse
2019-03-29T21:15:34Z

On Thu, Mar 28, 2019 at 01:33:42PM -0700, John Hubbard wrote:
> On 3/25/19 7:40 AM, jglisse@redhat.com wrote:
> > From: Jérôme Glisse <jglisse@redhat.com>
> >
> > To avoid random config build issue, select mmu notifier when HMM is
> > selected. In any cases when HMM get selected it will be by users that
> > will also wants the mmu notifier.
> >
> > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> > Acked-by: Balbir Singh <bsingharora@gmail.com>
> > Cc: Ralph Campbell <rcampbell@nvidia.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > ---
> > mm/Kconfig | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 25c71eb8a7db..0d2944278d80 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -694,6 +694,7 @@ config DEV_PAGEMAP_OPS
> >
> > config HMM
> > bool
> > + select MMU_NOTIFIER
> > select MIGRATE_VMA_HELPER
> >
> > config HMM_MIRROR
> >
>
> Yes, this is a good move, given that MMU notifiers are completely,
> indispensably part of the HMM design and implementation.
>
> The alternative would also work, but it's not quite as good. I'm
> listing it in order to forestall any debate:
>
> config HMM
> bool
> + depends on MMU_NOTIFIER
> select MIGRATE_VMA_HELPER
>
> ...and "depends on" versus "select" is always a subtle question. But in
> this case, I'd say that if someone wants HMM, there's no advantage in
> making them know that they must first ensure MMU_NOTIFIER is enabled.
> After poking around a bit I don't see any obvious downsides either.

You can not depend on MMU_NOTIFIER it is one of the kernel config
option that is not selectable. So any config that need MMU_NOTIFIER
must select it.

>
> However, given that you're making this change, in order to avoid odd
> redundancy, you should also do this:
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 0d2944278d80..2e6d24d783f7 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -700,7 +700,6 @@ config HMM
> config HMM_MIRROR
> bool "HMM mirror CPU page table into a device page table"
> depends on ARCH_HAS_HMM
> - select MMU_NOTIFIER
> select HMM
> help
> Select HMM_MIRROR if you want to mirror range of the CPU page table of a

Because it is a select option no harm can come from that hence i do
not remove but i can remove it.

Cheers,
Jérôme