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

[Dan Williams]

On Mon, Dec 19, 2016 at 5:09 PM, Darrick J. Wong
<darrick.wong@xxxxxxxxxx> wrote:
> 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.

That's for device-dax, which is basically a poor man's PMEM_IMMUTABLE
via a character device interface. It's useful for cases where you want
an entire nvdimm namespace/volume in "no fs-metadata to worry about"
mode.  But, for sub-allocations of a namespace and support for
existing tooling, PMEM_IMMUTABLE is much more usable.