Re: [PATCH v15 4/7] mm: Introduce Reported pages

[Alexander Duyck]

On Mon, 2019-12-16 at 05:17 -0500, Nitesh Narayan Lal wrote:
> On 12/5/19 11:22 AM, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > 
> > In order to pave the way for free page reporting in virtualized
> > environments we will need a way to get pages out of the free lists and
> > identify those pages after they have been returned. To accomplish this,
> > this patch adds the concept of a Reported Buddy, which is essentially
> > meant to just be the Uptodate flag used in conjunction with the Buddy
> > page type.
> 
> [...]
> 
> > +enum {
> > +	PAGE_REPORTING_IDLE = 0,
> > +	PAGE_REPORTING_REQUESTED,
> > +	PAGE_REPORTING_ACTIVE
> > +};
> > +
> > +/* request page reporting */
> > +static void
> > +__page_reporting_request(struct page_reporting_dev_info *prdev)
> > +{
> > +	unsigned int state;
> > +
> > +	/* Check to see if we are in desired state */
> > +	state = atomic_read(&prdev->state);
> > +	if (state == PAGE_REPORTING_REQUESTED)
> > +		return;
> > +
> > +	/*
> > +	 *  If reporting is already active there is nothing we need to do.
> > +	 *  Test against 0 as that represents PAGE_REPORTING_IDLE.
> > +	 */
> > +	state = atomic_xchg(&prdev->state, PAGE_REPORTING_REQUESTED);
> > +	if (state != PAGE_REPORTING_IDLE)
> > +		return;
> > +
> > +	/*
> > +	 * Delay the start of work to allow a sizable queue to build. For
> > +	 * now we are limiting this to running no more than once every
> > +	 * couple of seconds.
> > +	 */
> > +	schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
> > +}
> > +
> 
> I think you recently switched to using an atomic variable for maintaining page
> reporting status as I was doing in v12.
> Which is good, as we will not have a disagreement on it now.

There is still some differences between our approaches if I am not
mistaken. Specifically I have code in place so that any requests to report
while we are actively working on reporting will trigger another pass being
scheduled after we completed. I still believe you were lacking any logic
like that as I recall.

> On a side note, apologies for not getting involved actively in the
> discussions/reviews as I am on PTO.

No problem. I've been mostly looking for input from the core MM
maintainers anyway as we sorted most of the virtualization pieces some
time ago.