Re: [PATCH 3/5] vfs: add a zero-initialization mode to fallocate

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Sep 23, 2021 at 06:21:19PM -0700, Jane Chu wrote:
> 
> On 9/23/2021 6:18 PM, Dan Williams wrote:
> > On Thu, Sep 23, 2021 at 3:54 PM Dave Chinner <david@fromorbit.com> wrote:
> > > 
> > > On Wed, Sep 22, 2021 at 10:42:11PM -0700, Dan Williams wrote:
> > > > On Wed, Sep 22, 2021 at 7:43 PM Dan Williams <dan.j.williams@intel.com> wrote:
> > > > > 
> > > > > On Wed, Sep 22, 2021 at 6:42 PM Dave Chinner <david@fromorbit.com> wrote:
> > > > > [..]
> > > > > > Hence this discussion leads me to conclude that fallocate() simply
> > > > > > isn't the right interface to clear storage hardware poison state and
> > > > > > it's much simpler for everyone - kernel and userspace - to provide a
> > > > > > pwritev2(RWF_CLEAR_HWERROR) flag to directly instruct the IO path to
> > > > > > clear hardware error state before issuing this user write to the
> > > > > > hardware.
> > > > > 
> > > > > That flag would slot in nicely in dax_iomap_iter() as the gate for
> > > > > whether dax_direct_access() should allow mapping over error ranges,
> > > > > and then as a flag to dax_copy_from_iter() to indicate that it should
> > > > > compare the incoming write to known poison and clear it before
> > > > > proceeding.
> > > > > 
> > > > > I like the distinction, because there's a chance the application did
> > > > > not know that the page had experienced data loss and might want the
> > > > > error behavior. The other service the driver could offer with this
> > > > > flag is to do a precise check of the incoming write to make sure it
> > > > > overlaps known poison and then repair the entire page. Repairing whole
> > > > > pages makes for a cleaner implementation of the code that tries to
> > > > > keep poison out of the CPU speculation path, {set,clear}_mce_nospec().
> > > > 
> > > > This flag could also be useful for preadv2() as there is currently no
> > > > way to read the good data in a PMEM page with poison via DAX. So the
> > > > flag would tell dax_direct_access() to again proceed in the face of
> > > > errors, but then the driver's dax_copy_to_iter() operation could
> > > > either read up to the precise byte offset of the error in the page, or
> > > > autoreplace error data with zero's to try to maximize data recovery.
> > > 
> > > Yes, it could. I like the idea - say RWF_IGNORE_HWERROR - to read
> > > everything that can be read from the bad range because it's the
> > > other half of the problem RWF_RESET_HWERROR is trying to address.
> > > That is, the operation we want to perform on a range with an error
> > > state is -data recovery-, not "reinitialisation". Data recovery
> > > requires two steps:
> > > 
> > > - "try to recover the data from the bad storage"; and
> > > - "reinitialise the data and clear the error state"
> > > 
> > > These naturally map to read() and write() operations, not
> > > fallocate(). With RWF flags they become explicit data recovery
> > > operations, unlike fallocate() which needs to imply that "writing
> > > zeroes" == "reset hardware error state". While that reset method
> > > may be true for a specific pmem hardware implementation it is not a
> > > requirement for all storage hardware. It's most definitely not a
> > > requirement for future storage hardware, either.
> > > 
> > > It also means that applications have no choice in what data they can
> > > use to reinitialise the damaged range with because fallocate() only
> > > supports writing zeroes. If we've recovered data via a read() as you
> > > suggest we could, then we can rebuild the data from other redundant
> > > information and immediately write that back to the storage, hence
> > > repairing the fault.
> > > 
> > > That, in turn, allows the filesystem to turn the RWF_RESET_HWERROR
> > > write into an exclusive operation and hence allow the
> > > reinitialisation with the recovered/repaired state to run atomically
> > > w.r.t. all other filesystem operations.  i.e. the reset write
> > > completes the recovery operation instead of requiring separate
> > > "reset" and "write recovered data into zeroed range" steps that
> > > cannot be executed atomically by userspace...
> > 
> > /me nods
> > 
> > Jane, want to take a run at patches for this ^^^?
> > 
> 
> Sure, I'll give it a try.
> 
> Thank you all for the discussions!

Cool, thank you!

--D

> 
> -jane
> 
>