Re: [PATCH] mm, isolation: avoid checking unmovable pages across pageblock boundary

[David Hildenbrand <david@redhat.com>]

On 14.09.20 17:00, Michal Hocko wrote:
> On Tue 08-09-20 10:30:35, David Hildenbrand wrote:
> [...]
>> - gigantic pages (alloc_contig_range()) never use ranges with holes
>> - CMA (alloc_contig_range()) never uses ranges with holes.
> 
> Where is that enforced?
> 

You mean with CMA? I think the way CMA obtains regions from memblock
would not allow for that. (allocating something that's not actually
there does not make too much sense)

For example, we do a couple of unchecked pfn_to_page() calls directly in
alloc_contig_range(), most prominently right at the beginning
(page_zone(pfn_to_page(start)).

Also, __test_page_isolated_in_pageblock() would never succeed in case we
have memory holes that have a valid memmap (PageReserved()).

I think we should update the documentation of alloc_contig_range()

"The PFN range must belong to a single zone and must not contain any holes."

And simplify all that code. Of course, assuming there isn't a CMA use
case with holes I am too blind to see.

-- 
Thanks,

David / dhildenb