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

[Jerome Glisse]

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@xxxxxxxxx wrote:
> > > > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > > > [...]
> > > >  
> > > > +	/* 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 +	 *   - Range is no longer valid (munmap).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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