Re: [PATCH 0/6 RFC] Mapping range lock

[Jan Kara]

On Wed 06-02-13 10:25:12, Dave Chinner wrote:
> On Mon, Feb 04, 2013 at 01:38:31PM +0100, Jan Kara wrote:
> > On Thu 31-01-13 16:07:57, Andrew Morton wrote:
> > > > c) i_mutex doesn't allow any paralellism of operations using it and some
> > > >    filesystems workaround this for specific cases (e.g. DIO reads). Using
> > > >    range locking allows for concurrent operations (e.g. writes, DIO) on
> > > >    different parts of the file. Of course, range locking itself isn't
> > > >    enough to make the parallelism possible. Filesystems still have to
> > > >    somehow deal with the concurrency when manipulating inode allocation
> > > >    data. But the range locking at least provides a common VFS mechanism for
> > > >    serialization VFS itself needs and it's upto each filesystem to
> > > >    serialize more if it needs to.
> > > 
> > > That would be useful to end-users, but I'm having trouble predicting
> > > *how* useful.
> >   As Zheng said, there are people interested in this for DIO. Currently
> > filesystems each invent their own tweaks to avoid the serialization at
> > least for the easiest cases.
> 
> The thing is, this won't replace the locking those filesystems use
> to parallelise DIO - it just adds another layer of locking they'll
> need to use. The locks filesystems like XFS use to serialise IO
> against hole punch also serialise against many more internal
> functions and so if these range locks don't have the same capability
> we're going to have to retain those locks even after the range locks
> are introduced. It basically means we're going to have two layers
> of range locks - one for IO sanity and atomicity, and then this
> layer just for hole punch vs mmap.
> 
> As i've said before, what we really need in XFS is IO range locks
> because we need to be able to serialise operations against IO in
> progress, not page cache operations in progress.
  Hum, I'm not sure I follow you here. So mapping tree lock + PageLocked +
PageWriteback serialize all IO for part of the file underlying the page.
I.e. at most one of truncate (punch hole), DIO, writeback, buffered write,
buffered read, page fault can run on that part of file. So how come it
doesn't provide enough serialization for XFS?

Ah, is it the problem that if two threads do overlapping buffered writes
to a file then we can end up with data mixed from the two writes (if we
didn't have something like i_mutex)?

> IOWs, locking at
> the mapping tree level does not provide the right exclusion
> semantics we need to get rid of the existing filesystem locking that
> allows concurrent IO to be managed.  Hence the XFS IO path locking
> suddenly because 4 locks deep:
> 
> 	i_mutex
> 	  XFS_IOLOCK_{SHARED,EXCL}
> 	    mapping range lock
> 	      XFS_ILOCK_{SHARED,EXCL}
> 
> That's because the buffered IO path uses per-page lock ranges and to
> provide atomicity of read vs write, read vs truncate, etc we still
> need to use the XFS_IOLOCK_EXCL to provide this functionality.
>
> Hence I really think we need to be driving this lock outwards to
> where the i_mutex currently sits, turning it into an *IO range
> lock*, and not an inner-level mapping range lock. i.e flattening the
> locking to:
> 
> 	io_range_lock(off, len)
> 	  fs internal inode metadata modification lock
  If I get you right, your IO range lock would be +- what current mapping
tree lock is for DIO and truncate but for buffered IO you'd want to release
it after the read / write is finished? That is possible to do if we keep the
per-page granularity but that's what you seem to have problems with.

> Yes, I know this causes problems with mmap and locking orders, but
> perhaps we should be trying to get that fixed first because it
> simplifies the whole locking schema we need for filesystems to
> behave sanely. i.e. shouldn't we be aiming to simplify things
> as we rework locking rather than make the more complex?
  Yes. I was looking at how we could free filesystems from mmap_sem locking
issues but as Al Viro put it, the use of mmap_sem is a mess which isn't
easy to untangle. I want to have a look at it but I fear it's going to be a
long run...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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