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

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Date:   Fri, 29 Mar 2019 17:15:29 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ralph Campbell <rcampbell@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 01/11] mm/hmm: select mmu notifier when selecting HMM

On Thu, Mar 28, 2019 at 01:33:42PM -0700, John Hubbard wrote:
> On 3/25/19 7:40 AM, jglisse@...hat.com wrote:
> > From: Jérôme Glisse <jglisse@...hat.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@...hat.com>
> > Acked-by: Balbir Singh <bsingharora@...il.com>
> > Cc: Ralph Campbell <rcampbell@...dia.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Dan Williams <dan.j.williams@...el.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