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

Dan Williams
2021-09-28T00:08:13Z

On Mon, Sep 27, 2021 at 2:58 PM Jane Chu <jane.chu@oracle.com> wrote:
>
> On 9/27/2021 2:07 PM, Dave Chinner wrote:
> > On Thu, Sep 23, 2021 at 06:35:16PM -0700, Darrick J. Wong wrote:
> >> 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!
> >
> > I'd like to propose a slight modification to the API: a single RWF
> > flag called RWF_RECOVER_DATA. On read, this means the storage tries
> > to read all the data it can from the range, and for the parts it
> > can't read data from (cachelines, sectors, whatever) it returns as
> > zeroes.
> >
> > On write, this means the errors over the range get cleared and the
> > user data provided gets written over the top of whatever was there.
> > Filesystems should perform this as an exclusive operation to that
> > range of the file.
> >
> > That way we only need one IOCB_RECOVERY flag, and for communicating
> > with lower storage layers (e.g. dm/md raid and/or hardware) only one
> > REQ_RECOVERY flag is needed in the bio.
> >
> > Thoughts?
>
> Sounds cleaner. Dan, your thoughts?

I like it. I was struggling with a way to unify the flag names, and
"recovery" is a good term for not surprising the caller with zeros.
I.e. don't use this flow to avoid errors, use this flow to maximize
data recovery.