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From: Jeff Moyer <jmoyer@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v5 2/8] drivers/pmem: Allow pmem_clear_poison() to accept arbitrary offset and len
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	(Dan Williams's message of "Mon, 24 Feb 2020 12:48:35 -0800")
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Dan Williams <dan.j.williams@intel.com> writes:

>> Let's just focus on reporting errors when we know we have them.
>
> That's the problem in my eyes. If software needs to contend with
> latent error reporting then it should always contend otherwise
> software has multiple error models to wrangle.

The only way for an application to know that the data has been written
successfully would be to issue a read after every write.  That's not a
performance hit most applications are willing to take.  And, of course,
the media can still go bad at a later time, so it only guarantees the
data is accessible immediately after having been written.

What I'm suggesting is that we should not complete a write successfully
if we know that the data will not be retrievable.  I wouldn't call this
adding an extra error model to contend with.  Applications should
already be checking for errors on write.

Does that make sense?  Are we talking past each other?

> Setting that aside we can start with just treating zeroing the same as
> the copy_from_iter() case and fail the I/O at the dax_direct_access()
> step.

OK.

> I'd rather have a separate op that filesystems can use to clear errors
> at block allocation time that can be enforced to have the correct
> alignment.

So would file systems always call that routine instead of zeroing, or
would they first check to see if there are badblocks?

-Jeff
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Subject: Re: [PATCH v5 2/8] drivers/pmem: Allow pmem_clear_poison() to accept arbitrary offset and len
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Dan Williams <dan.j.williams@intel.com> writes:

>> Let's just focus on reporting errors when we know we have them.
>
> That's the problem in my eyes. If software needs to contend with
> latent error reporting then it should always contend otherwise
> software has multiple error models to wrangle.

The only way for an application to know that the data has been written
successfully would be to issue a read after every write.  That's not a
performance hit most applications are willing to take.  And, of course,
the media can still go bad at a later time, so it only guarantees the
data is accessible immediately after having been written.

What I'm suggesting is that we should not complete a write successfully
if we know that the data will not be retrievable.  I wouldn't call this
adding an extra error model to contend with.  Applications should
already be checking for errors on write.

Does that make sense?  Are we talking past each other?

> Setting that aside we can start with just treating zeroing the same as
> the copy_from_iter() case and fail the I/O at the dax_direct_access()
> step.

OK.

> I'd rather have a separate op that filesystems can use to clear errors
> at block allocation time that can be enforced to have the correct
> alignment.

So would file systems always call that routine instead of zeroing, or
would they first check to see if there are badblocks?

-Jeff