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

Dan Williams
2016-12-21T21:33:40Z

On Wed, Dec 21, 2016 at 1:24 PM, Dave Chinner <david@fromorbit.com> wrote:
> On Wed, Dec 21, 2016 at 08:53:46AM -0800, Dan Williams wrote:
>> On Tue, Dec 20, 2016 at 4:40 PM, Darrick J. Wong
>> <darrick.wong@oracle.com> wrote:
>> > On Mon, Dec 19, 2016 at 05:18:40PM -0800, Dan Williams wrote:
>> >> On Mon, Dec 19, 2016 at 5:09 PM, Darrick J. Wong
>> >> <darrick.wong@oracle.com> 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.
>> >
>> > Well sure... but otoh I was thinking that it'd be pretty neat if we
>> > could use the same code regardless of whether the target file was a
>> > dax-device or an xfs file:
>> >
>> > fd = open("<some path>", O_RDWR);
>> > fstat(fd, &statbuf):
>> > fallocate(fd, FALLOC_FL_PMEM_IMMUTABLE, 0, statbuf.st_size);
>> > p = mmap(NULL, statbuf.st_size, PROT_READ | PROT_WRITE, fd, 0);
>> >
>> > *(p + 42) = 0xDEADBEEF;
>> > asm { clflush; } /* or whatever */
>> >
>> > ...so perhaps it would be a good idea to design the fallocate primitive
>> > around "prepare this fd for mmap-only pmem semantics" and let it the
>> > backend do zeroing and inode flag changes as necessary to make it
>> > happen. We'd need to do some bikeshedding about what the other falloc
>> > flags mean when we're dealing with pmem files and devices, but I think
>> > we should try to keep the userland presentation the same unless there's
>> > a really good reason not to.
>>
>> It would be interesting to use fallocate to size device-dax files...
>
> No. device-dax needs to die, not poison a bunch of existing file and
> block device APIs and behaviours with special snowflakes. Get
> DAX-enabled filesystems to do what you need, and get rid of this
> ugly, nasty hack.
>

Right, Christoph already killed fallocate for device-dax.

What we're looking for now is the next level of detail on how to get
started on PMEM_IMMUTABLE, as Ross asked a few messages back in this
thread, so we have a reasonable replacement for device-dax.