linux-kernel - Re: [PATCH] mm/Kconfig: additional help text for HMM

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Date:   Fri, 19 Jul 2019 13:38:28 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Pavel Machek <pavel@....cz>, Christoph Hellwig <hch@...radead.org>
CC:     Jason Gunthorpe <jgg@...pe.ca>, <john.hubbard@...il.com>,
        <SCheung@...dia.com>, <akpm@...ux-foundation.org>,
        <aneesh.kumar@...ux.vnet.ibm.com>, <benh@...nel.crashing.org>,
        <bsingharora@...il.com>, <dan.j.williams@...el.com>,
        <dnellans@...dia.com>, <ebaskakov@...dia.com>,
        <hannes@...xchg.org>, <jglisse@...hat.com>,
        <kirill.shutemov@...ux.intel.com>, <linux-kernel@...r.kernel.org>,
        <liubo95@...wei.com>, <mhairgrove@...dia.com>, <mhocko@...nel.org>,
        <paulmck@...ux.vnet.ibm.com>, <ross.zwisler@...ux.intel.com>,
        <sgutti@...dia.com>, <torvalds@...ux-foundation.org>,
        <vdavydov.dev@...il.com>
Subject: Re: [PATCH] mm/Kconfig: additional help text for HMM_MIRROR option

On 7/19/19 5:04 AM, Pavel Machek wrote:
> On Fri 2019-07-19 05:00:43, Christoph Hellwig wrote:
>> On Fri, Jul 19, 2019 at 08:48:53AM -0300, Jason Gunthorpe wrote:
>>> It is like MMU_NOTIFIERS, if something needs it, then it will select
>>> it.
>>>
>>> Maybe it should just be a hidden kconfig anyhow as there is no reason
>>> to turn it on without also turning on a using driver.
>>
>> We can't just select it due to the odd X86_64 || PPC64 dependency.
>>
>> Which also answers Pavels question:  you never really need it, as we
>> can only use it for optional functionality due to that.
> 
> Okay, just explain it in the help text :-)..
> 
> Alternatively... you can have WANT_HMM_MIRROR option drivers select,
> and option HMM_MIRROR which is yes if WANT_HMM_MIRROR && (X86_64 ||
> PPC64), no?
> 

Yes. This really should be a hidden option that just auto-enables. It's
not ideal to require people to both *find* HMM_MIRROR, *and* figure out 
that they need it. (I think it's just this way due to the history of how
HMM got merged--it started off as a kind of experimental sandbox, so
it had it's own config options, to avoid bothering anything else.)

I'll send out a new patch to just auto-select. The WANT_HMM_MIRROR
approach seems accurate, given the (X86_64 || PPC64) complication, probably
after -rc1 is ready (I don't see the ODP code using HMM yet, so that
must not have been merged yet.)

Longer term, I vaguely recall that there is no strong reason preventing 
HMM from being made to work on other arches, and am hoping that it was
just done this way to save development time. I don't want to leave it 
this way unless there's a good reason to.

thanks,
-- 
John Hubbard
NVIDIA