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

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Date:   Fri, 9 Sep 2016 08:40:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Xiao Guangrong <guangrong.xiao@...ux.intel.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 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.