Re: [PATCH 0/3] mm/fs: get PG_error out of the writeback reporting business

[Jeff Layton]

On Thu, 2017-03-09 at 12:02 +0100, Jan Kara wrote:
> On Thu 09-03-17 05:47:51, Jeff Layton wrote:
> > On Thu, 2017-03-09 at 10:04 +0100, Jan Kara wrote:
> > > On Wed 08-03-17 21:57:25, Ted Tso wrote:
> > > > On Tue, Mar 07, 2017 at 11:26:22AM +0100, Jan Kara wrote:
> > > > > On a more general note (DAX is actually fine here), I find the current
> > > > > practice of clearing page dirty bits on error and reporting it just once
> > > > > problematic. It keeps the system running but data is lost and possibly
> > > > > without getting the error anywhere where it is useful. We get away with
> > > > > this because it is a rare event but it seems like a problematic behavior.
> > > > > But this is more for the discussion at LSF.
> > > > 
> > > > I'm actually running into this in the last day or two because some MM
> > > > folks at $WORK have been trying to push hard for GFP_NOFS removal in
> > > > ext4 (at least when we are holding some mutex/semaphore like
> > > > i_data_sem) because otherwise it's possible for the OOM killer to be
> > > > unable to kill processes because they are holding on to locks that
> > > > ext4 is holding.
> > > > 
> > > > I've done some initial investigation, and while it's not that hard to
> > > > remove GFP_NOFS from certain parts of the writepages() codepath (which
> > > > is where we had been are running into problems), a really, REALLY big
> > > > problem is if any_filesystem->writepages() returns ENOMEM, it causes
> > > > silent data loss, because the pages are marked clean, and so data
> > > > written using buffered writeback goes *poof*.
> > > > 
> > > > I confirmed this by creating a test kernel with a simple patch such
> > > > that if the ext4 file system is mounted with -o debug, there was a 1
> > > > in 16 chance that ext4_writepages will immediately return with ENOMEM
> > > > (and printk the inode number, so I knew which inodes had gotten the
> > > > ENOMEM treatment).  The result was **NOT** pretty.
> > > > 
> > > > What I think we should strongly consider is at the very least, special
> > > > case ENOMEM being returned by writepages() during background
> > > > writeback, and *not* mark the pages clean, and make sure the inode
> > > > stays on the dirty inode list, so we can retry the write later.  This
> > > > is especially important since the process that issued the write may
> > > > have gone away, so there might not even be a userspace process to
> > > > complain to.  By converting certain page allocations (most notably in
> > > > ext4_mb_load_buddy) from GFP_NOFS to GFP_KMALLOC, this allows us to
> > > > release the i_data_sem lock and return an error.  This should allow
> > > > allow the OOM killer to do its dirty deed, and hopefully we can retry
> > > > the writepages() for that inode later.
> > > 
> > > Yeah, so if we can hope the error is transient, keeping pages dirty and
> > > retrying the write is definitely better option. For start we can say that
> > > ENOMEM, EINTR, EAGAIN, ENOSPC errors are transient, anything else means
> > > there's no hope of getting data to disk and so we just discard them. It
> > > will be somewhat rough distinction but probably better than what we have
> > > now.
> > > 
> > > 								Honza
> > 
> > I'm not sure about ENOSPC there. That's a return code that is
> > specifically expected to be returned by fsync. It seems like that ought
> > not be considered a transient error?
> 
> Yeah, for start we should probably keep ENOSPC as is to prevent surprises.
> Long term, we may need to make at least some ENOSPC situations behave as
> transient to make thin provisioned storage not loose data in case admin
> does not supply additional space fast enough (i.e., before ENOSPC is
> actually hit).
> 

Maybe we need a systemwide (or fs-level) tunable that makes ENOSPC a
transient error? Just have it hang until we get enough space when that
tunable is enabled?

> EIO is actually in a similar bucket although probably more on the "hard
> failure" side - I can imagine there can by types of storage and situations
> where the loss of connectivity to the storage is only transient. But for
> start I would not bother with this.
> 
> 								Honza

I don't see what we can reasonably do with -EIO other than return a hard
error. If we want to deal with loss of connectivity to storage as a
transient failure, I think that we'd need to ensure that the lower
layers return more distinct error codes in those cases (ENODEV or ENXIO
maybe? Or declare a new kernel-internal code -- EDEVGONE?).

In any case, I think that the basic idea of marking certain
writepage/writepages/launder_page errors as transient might be a
reasonable approach to handling this sanely.

The problem with all of this though is that we have a pile of existing
code that will likely need to be reworked for the new error handling. I
expect that we'll have to walk all of the
writepage/writepages/launder_page implementations and fix them up one by
one once we sort out the rules for this.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>