Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

[Linus Torvalds]

On Thu, Mar 15, 2018 at 11:21 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> Well, pfn_valid says that a given pfn is backed by a real memory and it
> has a valid struct page backing it.

No, it really just says the latter. There should be a "struct page" for it.

It may not be "real memory". The struct page might have it marked
reserved or something. But at least you should be able to get to the
"struct page" and look at it.

                   Linus