linux-kernel - Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page table helpers

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Date:   Fri, 9 Aug 2019 16:05:07 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Steven Price <Steven.Price@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Vineet Gupta <vgupta@...opsys.com>,
        James Hogan <jhogan@...nel.org>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-snps-arc@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page
 table helpers



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()
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 ?