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

[Jan Kara <jack@suse.cz>]

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).

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
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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