linux-kernel - Re: [PATCH] vmap(): don't allow invalid pages

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Date:   Wed, 26 Jan 2022 02:50:51 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Yury Norov <yury.norov@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] vmap(): don't allow invalid pages

On Wed, Jan 19, 2022 at 01:28:14PM +0000, Robin Murphy wrote:
> Is it page_to_pfn() guaranteed to work without blowing up if page is invalid
> in the first place? Looking at the CONFIG_SPARSEMEM case I'm not sure that's
> true...

Something that all the ARM people weighing in on this don't understand
is that basically nobody uses SPARSEMEM without SPARSEMEM_VMEMMAP.
So all this complicated code to do page_to_pfn() is never tested.
Real users all do a simple subtraction and so the simple pfn_valid()
works fine.