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

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Jan Kara <jack@suse.cz>
Cc: akpm@linux-foundation.org,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	mpe@ellerman.id.au, Ross Zwisler <zwisler@kernel.org>,
	"Oliver O'Halloran" <oohall@gmail.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default
Date: Thu, 28 Feb 2019 18:02:53 +0530	[thread overview]
Message-ID: <2452a0b6-a90e-52b8-cb9f-0b5a7fafbe64@linux.ibm.com> (raw)
In-Reply-To: <20190228094011.GB22210@quack2.suse.cz>

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 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 <aneesh.kumar@linux.ibm.com>
> 
> 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