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

[Aneesh Kumar K.V]

On 2/28/19 3:10 PM, Jan Kara wrote:
> On Thu 28-02-19 14:05:22, Aneesh Kumar K.V wrote:
> > Add a flag to indicate the ability to do huge pageÂdax mapping. On architecture
> > like ppc64, the hypervisor can disable huge page support in the guest. In
> > such a case, we should not enable huge page dax mapping. This patch adds
> > a flag which the architecture code will update to indicate huge page
> > dax mapping support.
> >
> > Architectures mostly do transparent_hugepage_flag = 0; if they can't
> > do hugepages. That also takes care of disabling dax hugepage mapping
> > with this change.
> >
> > Without this patch we get the below error with kvm on ppc64.
> >
> > [  118.849975] lpar: Failed hash pte insert with error -4
> >
> > NOTE: The patch also use
> >
> > echo never > /sys/kernel/mm/transparent_hugepage/enabled
> > to disable dax huge page mapping.
> >
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>
> Added Dan to CC for opinion. I kind of fail to see why you don't use
> TRANSPARENT_HUGEPAGE_FLAG for this. I know that technically DAX huge pages
> and normal THPs are different things but so far we've tried to avoid making
> that distinction visible to userspace.


I would also like to use the same flag. Was not sure whether it was ok. 
In fact that is one of the reason I hooked this to 
/sys/kernel/mm/transparent_hugepage/enabled. If we are ok with using 
same flag, we can kill the vma_is_dax() check completely.


-aneesh