Re: [PATCH 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page()

[Baolin Wang]

On 4/23/2023 1:19 PM, Mike Rapoport wrote:
> Hi,
>
> On Sat, Apr 22, 2023 at 06:15:18PM +0800, Baolin Wang wrote:
> > Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which
> > checks whether the given zone contains holes, and uses pfn_to_online_page()
> > to validate if the start pfn is online and valid, as well as using pfn_valid()
> > to validate the end pfn.
> >
> > However, though the start pfn of a pageblock is valid, it can not always
> > guarantee the end pfn of the pageblock is also valid (may be holes) in some
> > cases. For example, if the pageblock order is MAX_ORDER - 1, which will fall
>
> Nit: in the current mm tree the default pageblock order is MAX_ORDER.

Ah, yes, will change in next version.

> > into 2 sub-sections, and the end pfn of the pageblock may be hole even though
> > the start pfn is online and valid.
> >
> > This did not break anything until now, but the zone continuous is fragile
> > in this possible scenario. So as previous discussion[1], it is better to
> > add some comments to explain this possible issue in case there are some
> > future pfn walkers that rely on this.
> >
> > [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/page_alloc.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 6457b64fe562..dc4005b32ae0 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1502,6 +1502,14 @@ void __free_pages_core(struct page *page, unsigned int order)
> >    * interleaving within a single pageblock. It is therefore sufficient to check
> >    * the first and last page of a pageblock and avoid checking each individual
> >    * page in a pageblock.
> > + *
> > + * Note: if the start pfn of a pageblock is valid, but it can not always guarantee
> > + * the end pfn of the pageblock is also valid (may be holes) in some cases. For
> > + * example, if the pageblock order is MAX_ORDER - 1, which will fall into 2
> > + * sub-sections, and the end pfn of the pageblock may be hole even though the
> > + * start pfn is online and valid. This did not break anything until now, but be
> > + * careful this possible issue when checking if the whole pfns are valid of a
>
> careful about ...

OK. Thanks for reviewing.