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Subject: Re: [PATCH v2 01/11] mm/hmm: select mmu notifier when selecting HMM
To:     Jerome Glisse <jglisse@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>
References: <20190325144011.10560-1-jglisse@redhat.com>
 <20190325144011.10560-2-jglisse@redhat.com>
 <d4889f44-0cc5-3ef6-deeb-7302c93c1f90@nvidia.com>
 <20190329211529.GA6124@redhat.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <c05dabd3-dd06-74d9-951c-ae409da31cd5@nvidia.com>
Date:   Fri, 29 Mar 2019 14:42:58 -0700
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On 3/29/19 2:15 PM, Jerome Glisse wrote:
[...]
>> 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.
> 

aha, thanks for explaining that point about the non-user-selectable items,
I wasn't aware of that. (I had convinced myself that those were set by
hard-coding a choice in one of the Kconfig files.)

>>
>> 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.
> 

Yes, this is just a tiny housecleaning point, not anything earthshaking.

thanks,
-- 
John Hubbard
NVIDIA