Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 03, 2017 at 06:28:38AM -0400, Jeff Layton wrote:
> On Mon, 2017-04-03 at 14:25 +1000, NeilBrown wrote:
> > Also I think that EIO should always over-ride ENOSPC as the possible
> > responses are different.  That probably means you need a separate seq
> > number for each, which isn't ideal.
> > 
> 
> I'm not quite convinced that it's really useful to do anything but
> report the latest error.
> 
> But...if we did need to prefer one over another, could we get away with
> always reporting -EIO once that error occurs? If so, then we'd still
> just need a single sequence counter.

I wonder whether it's even worth supporting both EIO and ENOSPC for a
writeback problem.  If I understand correctly, at the time of write(),
filesystems check to see if they have enough blocks to satisfy the
request, so ENOSPC only comes up in the writeback context for thinly
provisioned devices.

Programs have basically no use for the distinction.  In either case,
the situation is the same.  The written data is safely in RAM and cannot
be written to the storage.  If one were to make superhuman efforts,
one could mmap the file and write() it to a different device, but that
is incredibly rare.  For most programs, the response is to just die and
let the human deal with the corrupted file.

>From a sysadmin point of view, of course the situation is different,
and the remedy is different, but they should be getting that information
through a different mechanism than monitoring the errno from every
system call.

If we do want to continue to support both EIO and ENOSPC from writeback,
then let's have EIO override ENOSPC as an error.  ie if an ENOSPC comes
in after an EIO is set, it only bumps the counter and applications will
see EIO, not ENOSPC on fresh calls to fsync().