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

[Matthew Wilcox]

On Tue, Apr 04, 2017 at 01:03:22PM +1000, NeilBrown wrote:
> On Mon, Apr 03 2017, Jeff Layton wrote:
> 
> > On Mon, 2017-04-03 at 12:16 -0700, Matthew Wilcox wrote:
> >> So, OK, that makes sense, we should keep allowing filesystems to report
> >> ENOSPC as a writeback error.  But I think much of the argument below
> >> still holds, and we should continue to have a prior EIO to be reported
> >> over a new ENOSPC (even if the program has already consumed the EIO).
> >
> > I'm fine with that (though I'd like Neil's thoughts before we decide
> > anything) there.
> 
> I'd like there be a well defined time when old errors were forgotten.
> It does make sense for EIO to persist even if ENOSPC or EDQUOT is
> received, but not forever.
> Clearing the remembered errors when put_write_access() causes
> i_writecount to reach zero is one option (as suggested), but I'm not
> sure I'm happy with it.
> 
> Local filesystems, or network filesystems which receive strong write
> delegations, should only ever return EIO to fsync.  We should
> concentrate on them first, I think.  As there is only one possible
> error, the seq counter is sufficient to "clear" it once it has been
> reported to fsync() (or write()?).
> 
> Other network filesystems could return a whole host of errors: ENOSPC
> EDQUOT ESTALE EPERM EFBIG ...
> Do we want to limit exactly which errors are allowed in generic code, or
> do we just support EIO generically and expect the filesystem to sort out
> the details for anything else?

I'd like us to focus on our POSIX compliance here and not return
arbitrary errors.  The relevant pages are here:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/fsync.html
http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html
http://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html

For close(), we have to map every error to EIO.
For fsync(), we can return any error that write() could have.  That limits
us to:

EFBIG ENOSPC EIO ENOBUFS ENXIO

I think EFBIG really isn't a writeback error; are there any network
filesystems that don't know the file size limit at the time they accept
the original write?  ENOBUFS seems like a transient error (*this* call to
fsync() failed, but the next one may succeed ... it's the equivalent of
ENOMEM).  ENXIO seems to me like it's a submission error, not a writeback
error.  So that leaves us with ENOSPC and EIO, as we have support today.

> One possible approach a filesystem could take is just to allow a single
> async writeback error.  After that error, all subsequent write()
> system calls become synchronous. As write() or fsync() is called on each
> file descriptor (which could possibly have sent the write which caused
> the error), an error is returned and that fact is counted.  Once we have
> returned as many errors as there are open file descriptors
> (i_writecount?), and have seen a successful write, the filesystem
> forgets all recorded errors and switches back to async writes (for that
> inode).   NFS does this switch-to-sync-on-error.  See nfs_need_check_write().
> 
> The "which could possibly have sent the write which caused the error" is
> an explicit reference to NFS.  NFS doesn't use the AS_EIO/AS_ENOSPC
> flags to return async errors.  It allocates an nfs_open_context for each
> user who opens a given inode, and stores an error in there.  Each dirty
> pages is associated with one of these, so errors a sure to go to the
> correct user, though not necessarily the correct fd at present.

... and you need the nfs_open_context in order to use the correct
credentials when writing a page to the server, correct?

> When we specify the new behaviour we should be careful to be as vague as
> possible while still saying what we need.  This allows filesystems some
> flexibility.
> 
>   If an error happens during writeback, the next write() or fsync() (or
>   ....) on the file descriptor to which data was written will return -1
>   with errno set to EIO or some other relevant error.  Other file
>   descriptors open on the same file may receive EIO or some other error
>   on a subsequent appropriate system call.
>   It should not be assumed that close() will return an error.  fsync()
>   must be called before close() if writeback errors are important to the
>   application.

Thanks for explaining what NFS does today.