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Date: Fri, 9 Aug 2019 06:52:02 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page
 table helpers
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To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Vlastimil Babka <vbabka@suse.cz>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Thomas Gleixner <tglx@linutronix.de>, Mike Rapoport <rppt@linux.vnet.ibm.com>, Jason Gunthorpe <jgg@ziepe.ca>, Dan Williams <dan.j.williams@intel.com>, Peter Zijlstra <peterz@infradead.org>, Michal Hocko <mhocko@kernel.org>, Mark Rutland <mark.rutland@arm.com>, Mark Brown <broonie@kernel.org>, Steven Price <Steven.Price@arm.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Masahiro Yamada <yamada.masahiro@socionext.com>, Kees Cook <keescook@chromium.org>, Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, Sri Krishna chowdary <schowdary@nvidia.com>, Dave Hansen <dave.hansen@intel.com>, Russell King - ARM Linux <linux@armlinux.org.uk>, Michael Ellerman <mpe@ellerman.id.au>, Paul Mackerras <paulus@samba.org>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, "David S. Miller" <davem@davemloft.net>, Vineet Gupta <vgupta@synopsys.com>, James Hogan <jhogan@kernel.org>, Paul Burton <paul.burton@mips.com>, Ralf Baechle <ralf@linux-mips.org>, 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

On Fri, Aug 09, 2019 at 04:05:07PM +0530, Anshuman Khandual wrote:
> On 08/09/2019 03:46 PM, 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?
> 
> We needed PFN to be PUD aligned for pfn_pud() and PMD aligned for mk_pmd().
> Now walking the kernel page table for a known symbol like kernel_init()

I didn't say to walk the kernel page table.  I said to call virt_to_pfn()
for a known symbol like kernel_init().

> as you had suggested earlier we might encounter page table page entries at PMD
> and PUD which might not be PMD or PUD aligned respectively. It seemed to me
> that alignment requirement is applicable only for mk_pmd() and pfn_pud()
> which create large mappings at those levels but that requirement does not
> exist for page table pages pointing to next level. Is not that correct ? Or
> I am missing something here ?

Just clear the bottom bits off the PFN until you get a PMD or PUD aligned
PFN.  It's really not hard.