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Date: Thu, 15 Mar 2018 19:35:06 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
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        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock
 alignment"
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On Thu 15-03-18 11:28:27, Linus Torvalds wrote:
> On Thu, Mar 15, 2018 at 11:21 AM, Michal Hocko <mhocko@kernel.org> 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.

You are right! A simple example could be non volatile memory. That is
certainly not RAM but it has pfn_valid memory. Sorry about the
confusion.
-- 
Michal Hocko
SUSE Labs