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[217.140.110.172]) by mx.google.com with ESMTP id v21si1386475ejk.387.2019.08.09.04.44.58 for ; Fri, 09 Aug 2019 04:44:58 -0700 (PDT) Received-SPF: pass (google.com: domain of mark.rutland@arm.com designates 217.140.110.172 as permitted sender) client-ip=217.140.110.172; Authentication-Results: mx.google.com; spf=pass (google.com: domain of mark.rutland@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=mark.rutland@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C12CD1596; Fri, 9 Aug 2019 04:44:57 -0700 (PDT) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1DD783F575; Fri, 9 Aug 2019 04:44:53 -0700 (PDT) Date: Fri, 9 Aug 2019 12:44:51 +0100 From: Mark Rutland To: Matthew Wilcox Cc: Anshuman Khandual , linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , Greg Kroah-Hartman , Thomas Gleixner , Mike Rapoport , Jason Gunthorpe , Dan Williams , Peter Zijlstra , Michal Hocko , Mark Brown , Steven Price , Ard Biesheuvel , Masahiro Yamada , Kees Cook , Tetsuo Handa , Sri Krishna chowdary , Dave Hansen , Russell King - ARM Linux , Michael Ellerman , Paul Mackerras , Martin Schwidefsky , Heiko Carstens , "David S. Miller" , Vineet Gupta , James Hogan , Paul Burton , Ralf Baechle , linux-snps-arc@lists.infradead.org, linux-mips@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page table helpers Message-ID: <20190809114450.GF48423@lakrids.cambridge.arm.com> References: <1565335998-22553-1-git-send-email-anshuman.khandual@arm.com> <20190809101632.GM5482@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190809101632.GM5482@bombadil.infradead.org> User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Aug 09, 2019 at 03:16:33AM -0700, Matthew Wilcox wrote: > On Fri, Aug 09, 2019 at 01:03:17PM +0530, Anshuman Khandual wrote: > > Should alloc_gigantic_page() be made available as an interface for general > > use in the kernel. The test module here uses very similar implementation from > > HugeTLB to allocate a PUD aligned memory block. Similar for mm_alloc() which > > needs to be exported through a header. > > Why are you allocating memory at all instead of just using some > known-to-exist PFNs like I suggested? IIUC the issue is that there aren't necessarily known-to-exist PFNs that are sufficiently aligned -- they may not even exist. For example, with 64K pages, a PMD covers 512M. The kernel image is (generally) smaller than 512M, and will be mapped at page granularity. In that case, any PMD entry for a kernel symbol address will point to the PTE level table, and that will only necessarily be page-aligned, as any P?D level table is only necessarily page-aligned. In the same configuration, you could have less than 512M of total memory, and none of this memory is necessarily aligned to 512M. So beyond the PTE level, I don't think you can guarantee a known-to-exist valid PFN. I also believe that synthetic PFNs could fail pfn_valid(), so that might cause us pain too... Thanks, Mark.