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[209.85.220.65]) by mx.google.com with SMTPS id v88sor6386470otb.110.2019.03.13.09.07.24 for (Google Transport Security); Wed, 13 Mar 2019 09:07:24 -0700 (PDT) Received-SPF: pass (google.com: domain of dan.j.williams@intel.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@intel-com.20150623.gappssmtp.com header.s=20150623 header.b=Ev3mjXd6; spf=pass (google.com: domain of dan.j.williams@intel.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=dan.j.williams@intel.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=intel.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc:content-transfer-encoding; bh=/XfqRx4tzKzsjCcb8QbRLqX/uBHxzCtpu1uDPCItsOs=; b=Ev3mjXd6En91xODIUX56pYwhJLO9DdmQ95Gif+qABOQYxSXLSsL3HdkZ996LpFM0Ok BNFXboh0vkQcZET1UNi2xWbfJOgLi7YJOBlDe07HQIOTkXhyuZZtxYx1Jr323nQBINw1 bCIVmxF4JR2efLcgr/ZOkQzDj5SYAbGOQud/xj7awoc+6yVUoor9qC0PAVErRf0dlx+t qM1zGEHB4PSIuwMXtFmThJuVWsfXkY8L4/6g612b2JAC90VycrbkkPGVtxjK+QjLV1tp zV3E3pIxFXDm4fJ1q4lhu5iiryNlMnlw3ajMFHOD83PMclLscUSXLEmMdZOnDZ62lqrK YZQA== X-Google-Smtp-Source: APXvYqzH9OHk6fwYw9vNd25aIvBEiA2sVSv1vjB2i+lT3GwQjeRBJM8aOKz1bJJDYo3s88am/imNu4byiUR9Jvcm8nc= X-Received: by 2002:a9d:224a:: with SMTP id o68mr16122221ota.214.1552493244500; Wed, 13 Mar 2019 09:07:24 -0700 (PDT) MIME-Version: 1.0 References: <20190228083522.8189-1-aneesh.kumar@linux.ibm.com> <20190228083522.8189-2-aneesh.kumar@linux.ibm.com> <87k1hc8iqa.fsf@linux.ibm.com> <20190306124453.126d36d8@naga.suse.cz> In-Reply-To: From: Dan Williams Date: Wed, 13 Mar 2019 09:07:13 -0700 Message-ID: Subject: Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default To: "Aneesh Kumar K.V" Cc: =?UTF-8?Q?Michal_Such=C3=A1nek?= , Oliver , Jan Kara , linux-nvdimm , Linux Kernel Mailing List , Linux MM , Ross Zwisler , Andrew Morton , linuxppc-dev , "Kirill A . Shutemov" Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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 Wed, Mar 6, 2019 at 4:46 AM Aneesh Kumar K.V wrote: > > On 3/6/19 5:14 PM, Michal Such=C3=A1nek wrote: > > On Wed, 06 Mar 2019 14:47:33 +0530 > > "Aneesh Kumar K.V" wrote: > > > >> Dan Williams writes: > >> > >>> On Thu, Feb 28, 2019 at 1:40 AM Oliver wrote: > >>>> > >>>> On Thu, Feb 28, 2019 at 7:35 PM Aneesh Kumar K.V > >>>> wrote: > > > >> Also even if the user decided to not use THP, by > >> echo "never" > transparent_hugepage/enabled , we should continue to ma= p > >> dax fault using huge page on platforms that can support huge pages. > > > > Is this a good idea? > > > > This knob is there for a reason. In some situations having huge pages > > can severely impact performance of the system (due to host-guest > > interaction or whatever) and the ability to really turn off all THP > > would be important in those cases, right? > > > > My understanding was that is not true for dax pages? These are not > regular memory that got allocated. They are allocated out of /dev/dax/ > or /dev/pmem*. Do we have a reason not to use hugepages for mapping > pages in that case? The problem with the transparent_hugepage/enabled interface is that it conflates performing compaction work to produce THP-pages with the ability to map huge pages at all. The compaction is a nop for dax because the memory is already statically allocated. If the administrator does not want dax to consume huge TLB entries then don't configure huge-page dax. If a hypervisor wants to force disable huge-page-configured device-dax instances after the fact it seems we need an explicit interface for that and not overload transparent_hugepage/enabled.