Re: [RFC PATCH 11/15] mm/page_reporting: report pages at section size instead of MAX_ORDER.

[Alexander Duyck]

On Mon, Aug 9, 2021 at 8:08 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> On 9 Aug 2021, at 10:12, Alexander Duyck wrote:
>
> > On Mon, Aug 9, 2021 at 12:25 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 05.08.21 21:02, Zi Yan wrote:
> >>> From: Zi Yan <ziy@xxxxxxxxxx>
> >>>
> >>> page_reporting_order was set to MAX_ORDER, which is always smaller than
> >>> a memory section size. An upcoming change will make MAX_ORDER larger
> >>> than a memory section size. Set page_reporting_order to
> >>> PFN_SECTION_SHIFT to match existing size assumption.
> >>>
> >>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> >>> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >>> Cc: linux-mm@xxxxxxxxx
> >>> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >>> ---
> >>>   mm/page_reporting.c | 3 ++-
> >>>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> >>> index 382958eef8a9..dc4a2d699862 100644
> >>> --- a/mm/page_reporting.c
> >>> +++ b/mm/page_reporting.c
> >>> @@ -11,7 +11,8 @@
> >>>   #include "page_reporting.h"
> >>>   #include "internal.h"
> >>>
> >>> -unsigned int page_reporting_order = MAX_ORDER;
> >>> +/* Set page_reporting_order at section size */
> >>> +unsigned int page_reporting_order = PFN_SECTION_SHIFT;
> >>>   module_param(page_reporting_order, uint, 0644);
> >>>   MODULE_PARM_DESC(page_reporting_order, "Set page reporting order");
> >>>
> >>>
> >>
> >> If you look closely, this is only a placeholder and will get overwritten
> >> in page_reporting_register(). I don't recall why we have the module
> >> parameter at all. Most probably, to adjust the reporting order after we
> >> already registered a user. Can't we just initialize that to 0 ?
> >
> > Yeah, it is pretty much there for debugging in the event that we are
> > on an architecture that is misconfigured.
>
> MAX_ORDER is changed to a boot time variable in Patch 15, thus cannot be used
> for page_reporting_order initialization after that.
>
> Thanks for David’s explanation. I will initialize page_reporting_order to 0
> and fix the commit message.

Rather than 0 it might be better to use (unsigned)-1 as that would
prevent page reporting from being able to run until the value is
overwritten.