Re: [LSF/MM TOPIC] The end of the DAX experiment

Ric Wheeler
2019-02-17T20:29:21Z

On 2/6/19 4:12 PM, Dan Williams wrote:
> Before people get too excited this isn't a proposal to kill DAX. The
> topic proposal is a discussion to resolve lingering open questions
> that currently motivate ext4 and xfs to scream "EXPERIMENTAL" when the
> current DAX facilities are enabled. The are 2 primary concerns to
> resolve. Enumerate the remaining features/fixes, and identify a path
> to implement it all without regressing any existing application use
> cases.
>
> An enumeration of remaining projects follows, please expand this list
> if I missed something:
>
> * "DAX" has no specific meaning by itself, users have 2 use cases for
> "DAX" capabilities: userspace cache management via MAP_SYNC, and page
> cache avoidance where the latter aspect of DAX has no current api to
> discover / use it. The project is to supplement MAP_SYNC with a
> MAP_DIRECT facility and MADV_SYNC / MADV_DIRECT to indicate the same
> dynamically via madvise. Similar to O_DIRECT, MAP_DIRECT would be an
> application hint to avoid / minimiize page cache usage, but no strict
> guarantee like what MAP_SYNC provides.

Sounds like a great topic to me. Having just gone through a new round of USENIX
paper reviews, it is interesting to see how many academic systems are being
pitched in this space (and most of them don't mention the kernel based xfs/ext4
with dax).

Regards,

Ric

>
> * Resolve all "if (dax) goto fail;" patterns in the kernel. Outside of
> longterm-GUP (a topic in its own right) the projects here are
> XFS-reflink and XFS-realtime-device support. DAX+reflink effectively
> requires a given physical page to be mapped into two different inodes
> at different (page->index) offsets. The challenge is to support
> DAX-reflink without violating any existing application visible
> semantics, the operating assumption / strawman to debate is that
> experimental status is not blanket permission to go change existing
> semantics in backwards incompatible ways.
>
> * Deprecate, but not remove, the DAX mount option. Too many flows
> depend on the option so it will never go away, but the facility is too
> coarse. Provide an option to enable MAP_SYNC and
> more-likely-to-do-something-useful-MAP_DIRECT on a per-directory
> basis. The current proposal is to allow this property to only be
> toggled while the directory is empty to avoid the complications of
> racing page invalidation with new DAX mappings.
>
>
> Secondary projects, i.e. important but I would submit are not in the
> critical path to removing the "experimental" designation:
> * Filesystem-integrated badblock management. Hook up the media error
> notifications from libnvdimm to the filesystem to allow for operations
> like "list files with media errors" and "enumerate bad file offsets on
> a granulatiy smaller than a page". Another consideration along these
> lines is to integrate machine-check-handling and dynamic error
> notification into a filesystem interface. I've heard complaints that
> the sigaction() based mechanism to receive BUS_MCEERR_* information,
> while sufficient for the "System RAM" use case, is not precise enough
> for the "Persistent Memory / DAX" use case where errors are repairable
> and sub-page error information is useful.
>
> * Userfaultfd for file-backed mappings and DAX
>
>
> Ideally all the usual DAX, persistent memory, and GUP suspects could
> be in the room to discuss this:
> * Jan Kara
> * Dave Chinner
> * Christoph Hellwig
> * Jeff Moyer
> * Johannes Thumshirn
> * Matthew Wilcox
> * John Hubbard
> * Jérôme Glisse
> * MM folks for the reflink vs 'struct page' vs Xarray considerations