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

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Date:   Mon, 19 Dec 2016 17:09:36 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jeff Moyer <jmoyer@...hat.com>,
        Yumei Huang <yuhuang@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Xiaof Guangrong <guangrong.xiao@...ux.intel.com>,
        KVM list <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Gleb Natapov <gleb@...nel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        mtosatti@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linux MM <linux-mm@...ck.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in
 /proc/self/smaps)

On Mon, Dec 19, 2016 at 02:11:49PM -0700, Ross Zwisler wrote:
> On Fri, Sep 16, 2016 at 03:54:05PM +1000, Nicholas Piggin wrote:
> <>
> > Definitely the first step would be your simple preallocated per
> > inode approach until it is shown to be insufficient.
> 
> Reviving this thread a few months later...
> 
> Dave, we're interested in taking a serious look at what it would take to get
> PMEM_IMMUTABLE working.  Do you still hold the opinion that this is (or could
> become, with some amount of work) a workable solution?
> 
> We're happy to do the grunt work for this feature, but we will probably need
> guidance from someone with more XFS experience.  With you out on extended leave
> the first half of 2017, who would be the best person to ask for this guidance?
> Darrick?

Yes, probably. :)

I think where we left off with this (on the XFS side) is some sort of
fallocate mode that would allocate blocks, zero them, and then set the
DAX and PMEM_IMMUTABLE on-disk inode flags.  After that, you'd mmap the
file and thereby gain the ability to control write persistents behavior
without having to worry about fs metadata updates.  As an added plus, I
think zeroing the pmem also clears media errors, or something like that.

<shrug> Is that a reasonable starting point?  My memory is a little foggy.

Hmm, I see Dan just posted something about blockdev fallocate.  I'll go
read that.

--D

> 
> Thanks,
> - Ross