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

[Dan Williams]

On Tue, Mar 19, 2019 at 1:45 AM Kirill A. Shutemov <kirill at shutemov.name> wrote:
>
> On Wed, Mar 13, 2019 at 09:07:13AM -0700, Dan Williams wrote:
> > On Wed, Mar 6, 2019 at 4:46 AM Aneesh Kumar K.V
> > <aneesh.kumar at linux.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 at linux.ibm.com> wrote:
> > > >
> > > >> Dan Williams <dan.j.williams at intel.com> writes:
> > > >>
> > > >>> On Thu, Feb 28, 2019 at 1:40 AM Oliver <oohall at gmail.com> wrote:
> > > >>>>
> > > >>>> On Thu, Feb 28, 2019 at 7:35 PM Aneesh Kumar K.V
> > > >>>> <aneesh.kumar at linux.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.
>
> That's not [entirely] true. transparent_hugepage/defrag gates heavy-duty
> compaction. We do only very limited compaction if it's not advised by
> transparent_hugepage/defrag.
>
> I believe DAX has to respect transparent_hugepage/enabled. Or not
> advertise its huge pages as THP. It's confusing for user.

What does "advertise its huge pages as THP" mean in practice? I think
it's confusing that DAX, a facility that bypasses System RAM, is
affected by a transparent_hugepage flag which is a feature for
combining System RAM pages into larger pages. For the same reason that
transparent_hugepage does not gate / control hugetlb operation is the
same reason that transparent_hugepage should not gate / control DAX. A
global setting to disable opportunistic large page mappings of
System-RAM makes sense, but I don't see why that should read on DAX?