linux-kernel - Re: [PATCH v2 2/5] mm: Teach pfn_to_online_page() to consider subsection validity

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Date:   Tue, 12 Jan 2021 11:48:23 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        Qian Cai <cai@....pw>, Michal Hocko <mhocko@...e.com>,
        vishal.l.verma@...el.com, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2 2/5] mm: Teach pfn_to_online_page() to consider
 subsection validity

On Tue, Jan 12, 2021 at 10:53:17AM +0100, David Hildenbrand wrote:
> That's not sufficient for alternative implementations of pfn_valid().
> 
> You still need some kind of pfn_valid(pfn) for alternative versions of
> pfn_valid(). Consider arm64 memory holes in the memmap. See their
> current (yet to be fixed/reworked) pfn_valid() implementation.
> (pfn_valid_within() is implicitly active on arm64)
> 
> Actually, I think we should add something like the following, to make
> this clearer (pfn_valid_within() is confusing)
> 
> #ifdef CONFIG_HAVE_ARCH_PFN_VALID
> 	/* We might have to check for holes inside the memmap. */
> 	if (!pfn_valid())
> 		return NULL;
> #endif

I have to confess that I was a bit confused by pfn_valid_within + HOLES_IN_ZONES
+ HAVE_ARCH_PFN_VALID.

At first I thought that we should stick with pfn_valid_within, as we also
depend on HOLES_IN_ZONES, so it could be that

 if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID))
  ...

would to too much work, as if CONFIG_HOLES_IN_ZONES was not set but an arch
pfn_valid was provided, we would perform unedeed checks.
But on a closer look, CONFIG_HOLES_IN_ZONES is set by default on arm64, and
on ia64 when SPARSEMEM is set, so looks fine.


-- 
Oscar Salvador
SUSE L3