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

[Gavin Shan]

On 6/25/21 11:19 AM, Alexander Duyck wrote:
> On Thu, Jun 24, 2021 at 4:46 PM Gavin Shan <gshan@xxxxxxxxxx> 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;
> > +
>
> An alternative to this would be to look at setting up some
> comparisons. I might add another variable and do something like:
> order = prdev->order ? : pageblock_order;
> if (order < page_reporting_order)
>      page_reporting_order = order;
>
> You could essentially do something similar in the previous patch but
> just use pageblock_order directly rather than having to add a local
> variable.
>
> That way if you need to still pull down the page reporting order you
> can do so without prdev->order or pageblock_order overwriting the
> value and pushing it back up.

Thanks, Alex. Lets do both in v5, which will be posted shortly.

Thanks,
Gavin