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

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Date:   Wed, 13 Mar 2019 09:07:13 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Michal Suchánek <msuchanek@...e.de>,
        Oliver <oohall@...il.com>, Jan Kara <jack@...e.cz>,
        linux-nvdimm <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 6, 2019 at 4:46 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> 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?

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.