linux-kernel - Re: [PATCH 07/36] HMM: add per mirror page table v3.

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Message-ID: <20150629145053.GB2173@gmail.com>
Date:	Mon, 29 Jun 2015 10:50:54 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	Mark Hairgrove <mhairgrove@...dia.com>
Cc:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"joro@...tes.org" <joro@...tes.org>, Mel Gorman <mgorman@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Airlie <airlied@...hat.com>,
	Brendan Conoboy <blc@...hat.com>,
	Joe Donohue <jdonohue@...hat.com>,
	Duncan Poole <dpoole@...dia.com>,
	Sherry Cheung <SCheung@...dia.com>,
	Subhash Gutti <sgutti@...dia.com>,
	John Hubbard <jhubbard@...dia.com>,
	Lucien Dunning <ldunning@...dia.com>,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Arvind Gopalakrishnan <arvindg@...dia.com>,
	Haggai Eran <haggaie@...lanox.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Liran Liss <liranl@...lanox.com>,
	Roland Dreier <roland@...estorage.com>,
	Ben Sander <ben.sander@....com>,
	Greg Stoner <Greg.Stoner@....com>,
	John Bridgman <John.Bridgman@....com>,
	Michael Mantor <Michael.Mantor@....com>,
	Paul Blinzer <Paul.Blinzer@....com>,
	Laurent Morichetti <Laurent.Morichetti@....com>,
	Alexander Deucher <Alexander.Deucher@....com>,
	Oded Gabbay <Oded.Gabbay@....com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Jatin Kumar <jakumar@...dia.com>
Subject: Re: [PATCH 07/36] HMM: add per mirror page table v3.

On Fri, Jun 26, 2015 at 08:02:03PM -0700, Mark Hairgrove wrote:
> On Fri, 26 Jun 2015, Jerome Glisse wrote:
> > On Thu, Jun 25, 2015 at 04:05:48PM -0700, Mark Hairgrove wrote:
> > > On Thu, 21 May 2015, j.glisse@...il.com wrote:
> > > > From: Jérôme Glisse <jglisse@...hat.com>
> > > > [...]
> > > >  
> > > > +	/* update() - update device mmu following an event.
> > > > +	 *
> > > > +	 * @mirror: The mirror that link process address space with the device.
> > > > +	 * @event: The event that triggered the update.
> > > > +	 * Returns: 0 on success or error code {-EIO, -ENOMEM}.
> > > > +	 *
> > > > +	 * Called to update device page table for a range of address.
> > > > +	 * The event type provide the nature of the update :
> > > > +	 *   - Range is no longer valid (munmap).
> > > > +	 *   - Range protection changes (mprotect, COW, ...).
> > > > +	 *   - Range is unmapped (swap, reclaim, page migration, ...).
> > > > +	 *   - Device page fault.
> > > > +	 *   - ...
> > > > +	 *
> > > > +	 * Thought most device driver only need to use pte_mask as it reflects
> > > > +	 * change that will happen to the HMM page table ie :
> > > > +	 *   new_pte = old_pte & event->pte_mask;
> > > 
> > > Documentation request: It would be useful to break down exactly what is 
> > > required from the driver for each event type here, and what extra 
> > > information is provided by the type that isn't provided by the pte_mask.
> > 
> > Mostly event tell you if you need to free or not the device page table for
> > the range, which is not something you can infer from the pte_mask reliably.
> > Difference btw migration and munmap for instance, same pte_mask but range
> > is still valid in the migration case it will just be backed by a new set of
> > pages.
> 
> Given that event->pte_mask and event->type provide redundant information, 
> are they both necessary?

Like said, you can not infer event->type from pte_mask but you can infer
pte_mask from event->type. The idea is behind providing pte_mask is that
simple driver can just use that with the iter walk and simply mask the HMM
page table entry they read ((*ptep) & pte_mask) to repopulate the device
page table.

So yes pte_mask is redundant but i think it will be useful for a range of
device driver.

Cheers,
Jérôme
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