Re: [PATCH v4 3/4] mm/page_reporting: Allow driver to specify reporting order

[Gavin Shan]

On 6/25/21 3:48 PM, Michael S. Tsirkin wrote:
> On Fri, Jun 25, 2021 at 09:47:09AM +0800, Gavin Shan wrote:
> > The page reporting order (threshold) is sticky to @pageblock_order
> > by default. The page reporting can never be triggered because the
> > freeing page can't come up with a free area like that huge. The
> > situation becomes worse when the system memory becomes heavily
> > fragmented.
> >
> > For example, the following configurations are used on ARM64 when 64KB
> > base page size is enabled. In this specific case, the page reporting
> > won't be triggered until the freeing page comes up with a 512MB free
> > area. That's hard to be met, especially when the system memory becomes
> > heavily fragmented.
> >
> >     PAGE_SIZE:          64KB
> >     HPAGE_SIZE:         512MB
> >     pageblock_order:    13       (512MB)
> >     MAX_ORDER:          14
> >
> > This allows the drivers to specify the page reporting order when the
> > page reporting device is registered. It falls back to @pageblock_order
> > if it's not specified by the driver. The existing users (hv_balloon
> > and virtio_balloon) don't specify it and @pageblock_order is still
> > taken as their page reporting order. So this shouldn't introduce any
> > functional changes.
> >
> > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>
> > ---
> >   include/linux/page_reporting.h | 3 +++
> >   mm/page_reporting.c            | 6 ++++++
> >   2 files changed, 9 insertions(+)
> >
> > diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
> > index 3b99e0ec24f2..fe648dfa3a7c 100644
> > --- a/include/linux/page_reporting.h
> > +++ b/include/linux/page_reporting.h
> > @@ -18,6 +18,9 @@ struct page_reporting_dev_info {
> >   
> >   	/* Current state of page reporting */
> >   	atomic_t state;
> > +
> > +	/* Minimal order of page reporting */
> > +	unsigned int order;
> >   };
> >   
> >   /* Tear-down and bring-up for page reporting devices */
> > diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> > index 34bf4d26c2c4..382958eef8a9 100644
> > --- a/mm/page_reporting.c
> > +++ b/mm/page_reporting.c
> > @@ -329,6 +329,12 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
> >   		goto err_out;
> >   	}
> >   
> > +	/*
> > +	 * Update the page reporting order if it's specified by driver.
> > +	 * Otherwise, it falls back to @pageblock_order.
> > +	 */
> > +	page_reporting_order = prdev->order ? : pageblock_order;
> > +
>
> Hmm. So on ARM achitectures with 64K pages, the command line parameter
> is silently ignored?
>
> Isn't this a problem?

It's fine as the command line parameter is used as debugging purpose.
Besides, it can be changed by writing to the following file:

   /sys/module/page_reporting/parameters/page_reporting_order


> >   	/* initialize state and work structures */
> >   	atomic_set(&prdev->state, PAGE_REPORTING_IDLE);
> >   	INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
> > --
> > 2.23.0

Thanks,
Gavin