Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page()

[Baolin Wang]

On 4/12/2023 7:25 PM, David Hildenbrand wrote:
> On 12.04.23 12:45, 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_valid()
> > to check if the end pfn is valid. However pfn_valid() can not make sure
> > the end pfn is not a hole if the size of a pageblock is larger than the
> > size of a sub-mem_section, since the struct page getting by pfn_to_page()
> > may represent a hole or an unusable page frame, which may cause incorrect
> > zone contiguous is set.
> >
> > Though another user of pageblock_pfn_to_page() in compaction seems work
> > well now, it is better to avoid scanning or touching these offline pfns.
> > So like commit 2d070eab2e82 ("mm: consider zone which is not fully
> > populated to have holes"), we should also use pfn_to_online_page() for
> > the end pfn to make sure it is a valid pfn with usable page frame.
> > Meanwhile the pfn_valid() for end pfn can be dropped now.
> >
> > Moreover we've already used pfn_to_online_page() for start pfn to make
> > sure it is online and valid, so the pfn_valid() for the start pfn is
> > unnecessary, drop it.
>
> pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.

Yes, the granularity of memory hotplug is a mem_section.

However, suppose the pageblock order is MAX_ORDER-1, and the size of a 
sub-section is 2M, that means a pageblock will fall into 2 sub 
mem-section, and if there is a hole in the zone, that means the 2nd sub 
mem-section can be invalid without setting subsection_map bitmap.

So the start is online can make sure the end pfn of a pageblock is 
online, but a valid start pfn can not make sure the end pfn is valid in 
the bitmap of ms->usage->subsection_map.

> The exception to this rule is when we have a mixture of ZONE_DEVICE and 
> ZONE_* within the same section.
>
> Then, indeed the end might not be online.
>
> BUT, if the end is valid (-> ZONE_DEVICE), then the zone_id will differ. 
> [let's ignore any races for now, up to this point they are mostly of 
> theoretical nature]
>
> So I don't think this change actually fixes something.
>
>
> Getting rid of the pfn_valid(start_pfn)  makes sense. Replacing the 

Yes, my motivation is try to optimize the __pageblock_pfn_to_page() 
which is hot when doing compaction, and I saw these pfn_valid() can be 
dropped.

> pfn_valid(end_pfn) by a pfn_to_online_page(end_pfn) could make that 
> function less efficient.
>
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/page_alloc.c | 7 +++----
> >   1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index d0eb280ec7e4..8076f519c572 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1512,9 +1512,6 @@ struct page *__pageblock_pfn_to_page(unsigned 
> > long start_pfn,
> >       /* end_pfn is one past the range we are checking */
> >       end_pfn--;
> > -    if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
> > -        return NULL;
> > -
> >       start_page = pfn_to_online_page(start_pfn);
> >       if (!start_page)
> >           return NULL;
> > @@ -1522,7 +1519,9 @@ struct page *__pageblock_pfn_to_page(unsigned 
> > long start_pfn,
> >       if (page_zone(start_page) != zone)
> >           return NULL;
> > -    end_page = pfn_to_page(end_pfn);
> > +    end_page = pfn_to_online_page(end_pfn);
> > +    if (!end_page)
> > +        return NULL;
> >       /* This gives a shorter code than deriving page_zone(end_page) */
> >       if (page_zone_id(start_page) != page_zone_id(end_page))