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Date:   Wed, 26 Jan 2022 02:50:51 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Yury Norov <yury.norov@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Alexey Klimov <aklimov@redhat.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] vmap(): don't allow invalid pages
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References: <20220118235244.540103-1-yury.norov@gmail.com>
 <f85b3cac-29e7-4179-e078-fd859040c294@arm.com>
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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.