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

Kirill A. Shutemov
2019-03-06T13:06:30Z

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@linux.ibm.com> wrote:
> >
> > > Dan Williams <dan.j.williams@intel.com> writes:
> > >
> > > > On Thu, Feb 28, 2019 at 1:40 AM Oliver <oohall@gmail.com> wrote:
> > > > >
> > > > > On Thu, Feb 28, 2019 at 7:35 PM Aneesh Kumar K.V
> > > > > <aneesh.kumar@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?

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

--
Kirill A. Shutemov