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

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Message-ID: <20190826131308.GA15933@bombadil.infradead.org>
Date:   Mon, 26 Aug 2019 06:13:08 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
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 Mon, Aug 26, 2019 at 08:07:13AM +0530, Anshuman Khandual wrote:
> On 08/09/2019 07:22 PM, Matthew Wilcox wrote:
> > 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.
> 
> As Mark pointed out earlier that might end up being just a synthetic PFN
> which might not even exist on a given system.

And why would that matter?