From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=CBkT=WF=vger.kernel.org=linux-mips-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no
	autolearn_force=no version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 99E6CC433FF
	for <linux-mips@archiver.kernel.org>; Fri,  9 Aug 2019 11:44:59 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [209.132.180.67])
	by mail.kernel.org (Postfix) with ESMTP id 79C4F2166E
	for <linux-mips@archiver.kernel.org>; Fri,  9 Aug 2019 11:44:59 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S2406516AbfHILo7 (ORCPT <rfc822;linux-mips@archiver.kernel.org>);
        Fri, 9 Aug 2019 07:44:59 -0400
Received: from foss.arm.com ([217.140.110.172]:46226 "EHLO foss.arm.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1726537AbfHILo6 (ORCPT <rfc822;linux-mips@vger.kernel.org>);
        Fri, 9 Aug 2019 07:44:58 -0400
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 <mark.rutland@arm.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>, 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 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
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)
Sender: linux-mips-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-mips.vger.kernel.org>
X-Mailing-List: linux-mips@vger.kernel.org

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.