linux-kernel - Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

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Message-ID: <20190306130621.qrwybv5dpv3bzyym@kshutemo-mobl1>
Date:   Wed, 6 Mar 2019 16:06:21 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Michal Suchánek <msuchanek@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Oliver <oohall@...il.com>, Jan Kara <jack@...e.cz>,
        linux-nvdimm@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Ross Zwisler <zwisler@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

On Wed, Mar 06, 2019 at 06:15:25PM +0530, Aneesh Kumar K.V wrote:
> On 3/6/19 5:14 PM, Michal Suchánek wrote:
> > On Wed, 06 Mar 2019 14:47:33 +0530
> > "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> wrote:
> > 
> > > Dan Williams <dan.j.williams@...el.com> writes:
> > > 
> > > > On Thu, Feb 28, 2019 at 1:40 AM Oliver <oohall@...il.com> wrote:
> > > > > 
> > > > > On Thu, Feb 28, 2019 at 7:35 PM Aneesh Kumar K.V
> > > > > <aneesh.kumar@...ux.ibm.com> wrote:
> > > Also even if the user decided to not use THP, by
> > > echo "never" > transparent_hugepage/enabled , we should continue to map
> > > 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?

Yes. Like when you don't want dax to compete for TLB with mission-critical
application (which uses hugetlb for instance).

-- 
 Kirill A. Shutemov