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

[Aneesh Kumar K.V]

On 3/6/19 5:14 PM, Michal SuchÃnek wrote:
> On Wed, 06 Mar 2019 14:47:33 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> wrote:
>
> > Dan Williams <dan.j.williams@xxxxxxxxx> writes:
> >
> > > On Thu, Feb 28, 2019 at 1:40 AM Oliver <oohall@xxxxxxxxx> wrote:
> > > > On Thu, Feb 28, 2019 at 7:35 PM Aneesh Kumar K.V
> > > > <aneesh.kumar@xxxxxxxxxxxxx> 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?

-aneesh