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

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Message-Id: <df01bf6e-84a1-53fb-bf0c-0957af2f79e1@linux.ibm.com>
Date:   Wed, 6 Mar 2019 18:15:25 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Michal Suchánek <msuchanek@...e.de>
Cc:     Dan Williams <dan.j.williams@...el.com>, Oliver <oohall@...il.com>,
        Jan Kara <jack@...e.cz>, linux-nvdimm@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Ross Zwisler <zwisler@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

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

-aneesh