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

[Xiao Guangrong]

On 09/09/2016 11:40 PM, Dan Williams wrote:
> On Fri, Sep 9, 2016 at 1:55 AM, Xiao Guangrong
> <guangrong.xiao@xxxxxxxxxxxxxxx> 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. :)