Re: [PATCH v3 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page()

[Michal Hocko <mhocko@suse.com>]

On Tue 25-04-23 09:27:23, Baolin Wang wrote:
> 
> 
> On 4/25/2023 8:22 AM, Huang, Ying wrote:
> > Baolin Wang <baolin.wang@linux.alibaba.com> writes:
[...]
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 6457b64fe562..bd124390c79b 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1502,6 +1502,15 @@ 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: the function may return non-NULL struct page even for a page block
> > > + * which contains a memory hole (i.e. there is no physical memory for a subset
> > > + * of the pfn range). For example, if the pageblock order is MAX_ORDER, 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 should be safe most of
> > > + * the time because struct pages are still zero pre-filled and pfn walkers
> > 
> > I don't think the pfn is just zero-filled even it's a hole.  Can you
> > confirm that?  In memmap_init() and memmap_init_zone_range(),
> > init_unavailable_range() is called to initialize the struct page.
> 
> Yes, what I mean is the page frames were initialized to zero firstly, and
> some fields were initialized to default value. The "zero pre-filled" seems
> confusing, may be change to "initialized"?

Huang Ying is correct. Holes should have struct pages initialized and
init_unavailable_range actually marks those pages reserved. Which
is really good because they mean "do not touch unless this page is
yours". For some reason I thought those struct pages are simply zero
filled. I was clearly wrong. Maybe it would be good to reference
init_unavailable_range in the comment so that it is easier to track the
whole code path.

Sorry about that!
-- 
Michal Hocko
SUSE Labs