Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

[Ira Weiny]

On Thu, Jun 13, 2019 at 10:25:55AM +1000, Dave Chinner wrote:
> On Wed, Jun 12, 2019 at 05:37:53AM -0700, Matthew Wilcox wrote:
> > On Sat, Jun 08, 2019 at 10:10:36AM +1000, Dave Chinner wrote:
> > > On Fri, Jun 07, 2019 at 11:25:35AM -0700, Ira Weiny wrote:
> > > > Are you suggesting that we have something like this from user space?
> > > > 
> > > > 	fcntl(fd, F_SETLEASE, F_LAYOUT | F_UNBREAKABLE);
> > > 
> > > Rather than "unbreakable", perhaps a clearer description of the
> > > policy it entails is "exclusive"?
> > > 
> > > i.e. what we are talking about here is an exclusive lease that
> > > prevents other processes from changing the layout. i.e. the
> > > mechanism used to guarantee a lease is exclusive is that the layout
> > > becomes "unbreakable" at the filesystem level, but the policy we are
> > > actually presenting to uses is "exclusive access"...
> > 
> > That's rather different from the normal meaning of 'exclusive' in the
> > context of locks, which is "only one user can have access to this at
> > a time".
> 
> 
> Layout leases are not locks, they are a user access policy object.
> It is the process/fd which holds the lease and it's the process/fd
> that is granted exclusive access.  This is exactly the same semantic
> as O_EXCL provides for granting exclusive access to a block device
> via open(), yes?
> 
> > As I understand it, this is rather more like a 'shared' or
> > 'read' lock.  The filesystem would be the one which wants an exclusive
> > lock, so it can modify the mapping of logical to physical blocks.
> 
> ISTM that you're conflating internal filesystem implementation with
> application visible semantics. Yes, the filesystem uses internal
> locks to serialise the modification of the things the lease manages
> access too, but that has nothing to do with the access policy the
> lease provides to users.
> 
> e.g. Process A has an exclusive layout lease on file F. It does an
> IO to file F. The filesystem IO path checks that Process A owns the
> lease on the file and so skips straight through layout breaking
> because it owns the lease and is allowed to modify the layout. It
> then takes the inode metadata locks to allocate new space and write
> new data.
> 
> Process B now tries to write to file F. The FS checks whether
> Process B owns a layout lease on file F. It doesn't, so then it
> tries to break the layout lease so the IO can proceed. The layout
> breaking code sees that process A has an exclusive layout lease
> granted, and so returns -ETXTBSY to process B - it is not allowed to
> break the lease and so the IO fails with -ETXTBSY.
> 
> i.e. the exclusive layout lease prevents other processes from
> performing operations that may need to modify the layout from
> performing those operations. It does not "lock" the file/inode in
> any way, it just changes how the layout lease breaking behaves.

Question: Do we expect Process A to get notified that Process B was attempting
to change the layout?

This changes the exclusivity semantics.  While Process A has an exclusive lease
it could release it if notified to allow process B temporary exclusivity.

Question 2: Do we expect other process' (say Process C) to also be able to map
and pin the file?  I believe users will need this and for layout purposes it is
ok to do so.  But this means that Process A does not have "exclusive" access to
the lease.

So given Process C has also placed a layout lease on the file.  Indicating
that it does not want the layout to change.  Both A and C need to be "broken"
by Process B to change the layout.  If there is no Process B; A and C can run
just fine with a "locked" layout.

Ira

> 
> Further, the "exclusiveness" of a layout lease is completely
> irrelevant to the filesystem that is indicating that an operation
> that may need to modify the layout is about to be performed. All the
> filesystem has to do is handle failures to break the lease
> appropriately.  Yes, XFS serialises the layout lease validation
> against other IO to the same file via it's IO locks, but that's an
> internal data IO coherency requirement, not anything to do with
> layout lease management.
> 
> Note that I talk about /writes/ here. This is interchangable with
> any other operation that may need to modify the extent layout of the
> file, be it truncate, fallocate, etc: the attempt to break the
> layout lease by a non-owner should fail if the lease is "exclusive"
> to the owner.
> 
> > The complication being that by default the filesystem has an exclusive
> > lock on the mapping, and what we're trying to add is the ability for
> > readers to ask the filesystem to give up its exclusive lock.
> 
> The filesystem doesn't even lock the "mapping" until after the
> layout lease has been validated or broken.
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx
>