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

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Date:   Fri, 19 Jul 2019 08:48:53 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Machek <pavel@....cz>
Cc:     Christoph Hellwig <hch@...radead.org>, 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, jhubbard@...dia.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 Fri, Jul 19, 2019 at 12:52:39PM +0200, Pavel Machek wrote:
> On Thu 2019-07-18 22:57:48, Christoph Hellwig wrote:
> > On Thu, Jul 18, 2019 at 06:32:53PM -0700, john.hubbard@...il.com wrote:
> > > +	  HMM_MIRROR provides a way to mirror ranges of the CPU page tables
> > > +	  of a process into a device page table. Here, mirror means "keep
> > > +	  synchronized". Prerequisites: the device must provide the ability
> > > +	  to write-protect its page tables (at PAGE_SIZE granularity), and
> > > +	  must be able to recover from the resulting potential page faults.
> > > +
> > > +	  Select HMM_MIRROR if you have hardware that meets the above
> > > +	  description. An early, partial list of such hardware is:
> > > +	  an NVIDIA GPU >= Pascal, Mellanox IB >= mlx5, or an AMD GPU.
> > 
> > Nevermind that the Nvidia support is stagaging and looks rather broken,
> > there is no Mellanox user of this either at this point.
> > 
> > But either way this has no business in a common kconfig help.  Just
> > drop the fine grained details and leave it to the overview.
> 
> I disagree here. This explains what kind of hardware this is for (very
> new). Partial list does not hurt, and I know that I probably don't
> need to enable this.
> 
> How else am I supposed to know if my computer needs page tables
> synchronized?

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.

Jason