Re: [PATCH v3 06/10] writeback: introduce super_operations->write_metadata

[Jan Kara <jack@suse.cz>]

On Wed 13-12-17 09:20:04, Dave Chinner wrote:
> On Tue, Dec 12, 2017 at 01:05:35PM -0500, Josef Bacik wrote:
> > On Tue, Dec 12, 2017 at 10:36:19AM +1100, Dave Chinner wrote:
> > > On Mon, Dec 11, 2017 at 04:55:31PM -0500, Josef Bacik wrote:
> > > > From: Josef Bacik <jbacik@fb.com>
> > > > 
> > > > Now that we have metadata counters in the VM, we need to provide a way to kick
> > > > writeback on dirty metadata.  Introduce super_operations->write_metadata.  This
> > > > allows file systems to deal with writing back any dirty metadata we need based
> > > > on the writeback needs of the system.  Since there is no inode to key off of we
> > > > need a list in the bdi for dirty super blocks to be added.  From there we can
> > > > find any dirty sb's on the bdi we are currently doing writeback on and call into
> > > > their ->write_metadata callback.
> > > > 
> > > > Signed-off-by: Josef Bacik <jbacik@fb.com>
> > > > Reviewed-by: Jan Kara <jack@suse.cz>
> > > > Reviewed-by: Tejun Heo <tj@kernel.org>
> > > > ---
> > > >  fs/fs-writeback.c                | 72 ++++++++++++++++++++++++++++++++++++----
> > > >  fs/super.c                       |  6 ++++
> > > >  include/linux/backing-dev-defs.h |  2 ++
> > > >  include/linux/fs.h               |  4 +++
> > > >  mm/backing-dev.c                 |  2 ++
> > > >  5 files changed, 80 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > > > index 987448ed7698..fba703dff678 100644
> > > > --- a/fs/fs-writeback.c
> > > > +++ b/fs/fs-writeback.c
> > > > @@ -1479,6 +1479,31 @@ static long writeback_chunk_size(struct bdi_writeback *wb,
> > > >  	return pages;
> > > >  }
> > > >  
> > > > +static long writeback_sb_metadata(struct super_block *sb,
> > > > +				  struct bdi_writeback *wb,
> > > > +				  struct wb_writeback_work *work)
> > > > +{
> > > > +	struct writeback_control wbc = {
> > > > +		.sync_mode		= work->sync_mode,
> > > > +		.tagged_writepages	= work->tagged_writepages,
> > > > +		.for_kupdate		= work->for_kupdate,
> > > > +		.for_background		= work->for_background,
> > > > +		.for_sync		= work->for_sync,
> > > > +		.range_cyclic		= work->range_cyclic,
> > > > +		.range_start		= 0,
> > > > +		.range_end		= LLONG_MAX,
> > > > +	};
> > > > +	long write_chunk;
> > > > +
> > > > +	write_chunk = writeback_chunk_size(wb, work);
> > > > +	wbc.nr_to_write = write_chunk;
> > > > +	sb->s_op->write_metadata(sb, &wbc);
> > > > +	work->nr_pages -= write_chunk - wbc.nr_to_write;
> > > > +
> > > > +	return write_chunk - wbc.nr_to_write;
> > > 
> > > Ok, writeback_chunk_size() returns a page count. We've already gone
> > > through the "metadata is not page sized" dance on the dirty
> > > accounting side, so how are we supposed to use pages to account for
> > > metadata writeback?
> > > 
> > 
> > This is just one of those things that's going to be slightly shitty.  It's the
> > same for memory reclaim, all of those places use pages so we just take
> > METADATA_*_BYTES >> PAGE_SHIFT to get pages and figure it's close enough.
> 
> Ok, so that isn't exactly easy to deal with, because all our
> metadata writeback is based on log sequence number targets (i.e. how
> far to push the tail of the log towards the current head). We've
> actually got no idea how pages/bytes actually map to a LSN target
> because while we might account a full buffer as dirty for memory
> reclaim purposes (up to 64k in size), we might have only logged 128
> bytes of it.
> 
> i.e. if we are asked to push 2MB of metadata and we treat that as
> 2MB of log space (i.e. push target of tail LSN + 2MB) we could have
> logged several tens of megabytes of dirty metadata in that LSN
> range and have to flush it all. OTOH, if the buffers are fully
> logged, then that same target might only flush 1.5MB of metadata
> once all the log overhead is taken into account.
> 
> So there's a fairly large disconnect between the "flush N bytes of
> metadata" API and the "push to a target LSN" that XFS uses for
> flushing metadata in aged order. I'm betting that extN and otehr
> filesystems might have similar mismatches with their journal
> flushing...

Well, for ext4 it isn't as bad since we do full block logging only. So if
we are asked to flush N pages, we can easily translate that to number of fs
blocks and flush that many from the oldest transaction.

Couldn't XFS just track how much it has cleaned (from reclaim perspective)
when pushing items from AIL (which is what I suppose XFS would do in
response to metadata writeback request) and just stop pushing when it has
cleaned as much as it was asked to?

> > > And, from what I can tell, if work->sync_mode = WB_SYNC_ALL or
> > > work->tagged_writepages is set, this will basically tell us to flush
> > > the entire dirty metadata cache because write_chunk will get set to
> > > LONG_MAX.
> > > 
> > > IOWs, this would appear to me to change sync() behaviour quite
> > > dramatically on filesystems where ->write_metadata is implemented.
> > > That is, instead of leaving all the metadata dirty in memory and
> > > just forcing the journal to stable storage, filesystems will be told
> > > to also write back all their dirty metadata before sync() returns,
> > > even though it is not necessary to provide correct sync()
> > > semantics....
> > 
> > Well for btrfs that's exactly what we have currently since it's just backed by
> > an inode.
> 
> Hmmmm. That explains a lot.
> 
> Seems to me that btrfs is the odd one out here, so I'm not sure a
> mechanism primarily designed for btrfs is going to work
> generically....

For record ext4 is currently behaving the way btrfs is as well (at least in
practice). We expose committed but not yet checkpointed transaction data
(equivalent of XFS's AIL AFAICT) in block device's page cache as dirty
buffers. Thus calls like sync_blockdev() which are called as part of
sync(2) will result in flushing all of those metadata buffers to disk
(although as you properly point out it is not strictly necessary for
correctness of sync(2)).

> > Obviously this is different for journaled fs'es, but I assumed that
> > in your case you would either not use this part of the infrastructure or simply
> > ignore WB_SYNC_ALL and use WB_SYNC_NONE as a way to be nice under memory
> > pressure or whatever.
> 
> I don't think that designing an interface with the assumption other
> filesystems will abuse it until it works for them is a great process
> to follow...
> 
> > > Mind you, writeback invocation is so convoluted now I could easily
> > > be mis-interpretting this code, but it does seem to me like this
> > > code is going to have some unintended behaviours....
> > > 
> > 
> > I don't think so, because right now this behavior is exactly what btrfs has
> > currently with it's inode setup.  I didn't really think the journaled use case
> > out since you guys are already rate limited by the journal.
> 
> We are?
> 
> XFS is rate limited by metadata writeback, not journal throughput.
> Yes, journal space is limited by the metadata writeback rate, but
> journalling itself is not the bottleneck.
> 
> > If you would want
> > to start using this stuff what would you like to see done instead?  Thanks,
> 
> If this is all about reacting to memory pressure, then writeback is
> not the mechanism that should drive this writeback. Reacting to
> memory pressure is what shrinkers are for, and XFS already triggers
> metadata writeback on memory pressure. Hence I don't see how this
> writeback mechanism would help us if we have to abuse it to infer
> "memory pressure occurring"
> 
> What I was hoping for was this interface to be a mechanism to drive
> periodic background metadata writeback from the VFS so that when we
> start to run out of memory the VFS has already started to ramp up
> the rate of metadata writeback so we don't have huge amounts of dirty
> metadata to write back during superblock shrinker based reclaim.

Yeah, that's where I'd like this patch set to end up as well and I believe
Josef is as well - he wants to prevent too much dirty metadata to get
accumulated after all and background writeback of metadata is a part of
that.

> i.e. it works more like dirty background data writeback, get's the
> amount of work to do from the amount of dirty metadata associated
> with the bdi and doesn't actually do anything when operations like
> sync() are run because there isn't a need to writeback metadata in
> those operations.
> 
> IOWs, treating metadata like it's one great big data inode doesn't
> seem to me to be the right abstraction to use for this - in most
> fileystems it's a bunch of objects with a complex dependency tree
> and unknown write ordering, not an inode full of data that can be
> sequentially written.
> 
> Maybe we need multiple ops with well defined behaviours. e.g.
> ->writeback_metadata() for background writeback, ->sync_metadata() for
> sync based operations. That way different filesystems can ignore the
> parts they don't need simply by not implementing those operations,
> and the writeback code doesn't need to try to cater for all
> operations through the one op. The writeback code should be cleaner,
> the filesystem code should be cleaner, and we can tailor the work
> guidelines for each operation separately so there's less mismatch
> between what writeback is asking and how filesystems track dirty
> metadata...

I agree that writeback for memory cleaning and writeback for data integrity
are two very different things especially for metadata. In fact for data
integrity writeback we already have ->sync_fs operation so there the
functionality gets duplicated. What we could do is that in
writeback_sb_inodes() we'd call ->write_metadata only when
work->for_kupdate or work->for_background is set. That way ->write_metadata
would be called only for memory cleaning purposes.

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

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