linux-kernel - Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps)

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Date:   Mon, 12 Sep 2016 14:00:07 +0800
From:   Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Gleb Natapov <gleb@...nel.org>,
        mtosatti@...hat.com, KVM list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Yumei Huang <yuhuang@...hat.com>,
        Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in
 /proc/self/smaps)



On 09/09/2016 11:40 PM, Dan Williams wrote:
> On Fri, Sep 9, 2016 at 1:55 AM, Xiao Guangrong
> <guangrong.xiao@...ux.intel.com> wrote:
> [..]
>>>
>>> Whether a persistent memory mapping requires an msync/fsync is a
>>> filesystem specific question.  This mincore proposal is separate from
>>> that.  Consider device-DAX for volatile memory or mincore() called on
>>> an anonymous memory range.  In those cases persistence and filesystem
>>> metadata are not in the picture, but it would still be useful for
>>> userspace to know "is there page cache backing this mapping?" or "what
>>> is the TLB geometry of this mapping?".
>>
>>
>> I got a question about msync/fsync which is beyond the topic of this thread
>> :)
>>
>> Whether msync/fsync can make data persistent depends on ADR feature on
>> memory
>> controller, if it exists everything works well, otherwise, we need to have
>> another
>> interface that is why 'Flush hint table' in ACPI comes in. 'Flush hint
>> table' is
>> particularly useful for nvdimm virtualization if we use normal memory to
>> emulate
>> nvdimm with data persistent characteristic (the data will be flushed to a
>> persistent storage, e.g, disk).
>>
>> Does current PMEM programming model fully supports 'Flush hint table'? Is
>> userspace allowed to use these addresses?
>
> If you publish flush hint addresses in the virtual NFIT the guest VM
> will write to them whenever a REQ_FLUSH or REQ_FUA request is sent to
> the virtual /dev/pmemX device.  Yes, seems straightforward to take a
> VM exit on those events and flush simulated pmem to persistent
> storage.
>

Thank you, Dan!

However REQ_FLUSH or REQ_FUA is handled in kernel space, okay, after following
up the discussion in this thread, i understood that currently filesystems have
not supported the case that usespace itself make data be persistent without
kernel's involvement. So that works.

Hmm, Does device-DAX support this case (make data be persistent without
msync/fsync)? I guess no, but just want to confirm it. :)