On 2/28/19 3:10 PM, Jan Kara wrote:

On Thu 28-02-19 14:05:22, Aneesh Kumar K.V wrote:

quoted

Add a flag to indicate the ability to do huge page dax 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 <redacted>

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