Re: Lease semantic proposal

[J. Bruce Fields]

On Wed, Oct 02, 2019 at 08:28:40AM -0400, Jeff Layton wrote:
> On Tue, 2019-10-01 at 11:17 -0700, Ira Weiny wrote:
> > On Mon, Sep 23, 2019 at 04:17:59PM -0400, Jeff Layton wrote:
> > > On Mon, 2019-09-23 at 12:08 -0700, Ira Weiny wrote:
> > > > Since the last RFC patch set[1] much of the discussion of supporting RDMA with
> > > > FS DAX has been around the semantics of the lease mechanism.[2]  Within that
> > > > thread it was suggested I try and write some documentation and/or tests for the
> > > > new mechanism being proposed.  I have created a foundation to test lease
> > > > functionality within xfstests.[3] This should be close to being accepted.
> > > > Before writing additional lease tests, or changing lots of kernel code, this
> > > > email presents documentation for the new proposed "layout lease" semantic.
> > > > 
> > > > At Linux Plumbers[4] just over a week ago, I presented the current state of the
> > > > patch set and the outstanding issues.  Based on the discussion there, well as
> > > > follow up emails, I propose the following addition to the fcntl() man page.
> > > > 
> > > > Thank you,
> > > > Ira
> > > > 
> > > > [1] https://lkml.org/lkml/2019/8/9/1043
> > > > [2] https://lkml.org/lkml/2019/8/9/1062
> > > > [3] https://www.spinics.net/lists/fstests/msg12620.html
> > > > [4] https://linuxplumbersconf.org/event/4/contributions/368/
> > > > 
> > > > 
> > > 
> > > Thank you so much for doing this, Ira. This allows us to debate the
> > > user-visible behavior semantics without getting bogged down in the
> > > implementation details. More comments below:
> > 
> > Thanks.  Sorry for the delay in response.  Turns out this email was in my
> > spam...  :-/  I'll need to work out why.
> > 
> > > > <fcntl man page addition>
> > > > Layout Leases
> > > > -------------
> > > > 
> > > > Layout (F_LAYOUT) leases are special leases which can be used to control and/or
> > > > be informed about the manipulation of the underlying layout of a file.
> > > > 
> > > > A layout is defined as the logical file block -> physical file block mapping
> > > > including the file size and sharing of physical blocks among files.  Note that
> > > > the unwritten state of a block is not considered part of file layout.
> > > > 
> > > > **Read layout lease F_RDLCK | F_LAYOUT**
> > > > 
> > > > Read layout leases can be used to be informed of layout changes by the
> > > > system or other users.  This lease is similar to the standard read (F_RDLCK)
> > > > lease in that any attempt to change the _layout_ of the file will be reported to
> > > > the process through the lease break process.  But this lease is different
> > > > because the file can be opened for write and data can be read and/or written to
> > > > the file as long as the underlying layout of the file does not change.
> > > > Therefore, the lease is not broken if the file is simply open for write, but
> > > > _may_ be broken if an operation such as, truncate(), fallocate() or write()
> > > > results in changing the underlying layout.
> > > > 
> > > > **Write layout lease (F_WRLCK | F_LAYOUT)**
> > > > 
> > > > Write Layout leases can be used to break read layout leases to indicate that
> > > > the process intends to change the underlying layout lease of the file.
> > > > 
> > > > A process which has taken a write layout lease has exclusive ownership of the
> > > > file layout and can modify that layout as long as the lease is held.
> > > > Operations which change the layout are allowed by that process.  But operations
> > > > from other file descriptors which attempt to change the layout will break the
> > > > lease through the standard lease break process.  The F_LAYOUT flag is used to
> > > > indicate a difference between a regular F_WRLCK and F_WRLCK with F_LAYOUT.  In
> > > > the F_LAYOUT case opens for write do not break the lease.  But some operations,
> > > > if they change the underlying layout, may.
> > > > 
> > > > The distinction between read layout leases and write layout leases is that
> > > > write layout leases can change the layout without breaking the lease within the
> > > > owning process.  This is useful to guarantee a layout prior to specifying the
> > > > unbreakable flag described below.
> > > > 
> > > > 
> > > 
> > > The above sounds totally reasonable. You're essentially exposing the
> > > behavior of nfsd's layout leases to userland. To be clear, will F_LAYOUT
> > > leases work the same way as "normal" leases, wrt signals and timeouts?
> > 
> > That was my intention, yes.
> >
> > > I do wonder if we're better off not trying to "or" in flags for this,
> > > and instead have a separate set of commands (maybe F_RDLAYOUT,
> > > F_WRLAYOUT, F_UNLAYOUT). Maybe I'm just bikeshedding though -- I don't
> > > feel terribly strongly about it.
> > 
> > I'm leaning that was as well.  To make these even more distinct from
> > F_SETLEASE.
> > 
> > > Also, at least in NFSv4, layouts are handed out for a particular byte
> > > range in a file. Should we consider doing this with an API that allows
> > > for that in the future? Is this something that would be desirable for
> > > your RDMA+DAX use-cases?
> > 
> > I don't see this.  I've thought it would be a nice thing to have but I don't
> > know of any hard use case.  But first I'd like to understand how this works for
> > NFS.
> > 
> 
> The NFSv4.1 spec allows the client to request the layouts for a
> particular range in the file:
> 
> https://tools.ietf.org/html/rfc5661#page-538
> 
> The knfsd only hands out whole-file layouts at present. Eventually we
> may want to make better use of segmented layouts, at which point we'd
> need something like a byte-range lease.
> 
> > > We could add a new F_SETLEASE variant that takes a struct with a byte
> > > range (something like struct flock).
> > 
> > I think this would be another reason to introduce F_[RD|WR|UN]LAYOUT as a
> > command.  Perhaps supporting smaller byte ranges could be added later?
> > 
> 
> I'd definitely not multiplex this over F_SETLEASE. An F_SETLAYOUT cmd
> would probably be sufficient, and maybe just reuse
> F_RDLCK/F_WRLCK/F_UNLCK for the iomode?
> 
> For the byte ranges, the catch there is that extending the userland
> interface for that later will be difficult.

Why would it be difficult?

> What I'd probably suggest
> (and what would jive with the way pNFS works) would be to go ahead and
> add an offset and length to the arguments and result (maybe also
> whence?).

Why not add new commands with range arguments later if it turns out to
be necessary?

--b.

> 
> The current kernel implementation could be free to deliver a larger
> range than requested and only hand out full-file layouts for now.
> Eventually we could rework the internals to allow for byte-range layout
> leases.
> 
> I think this means that you'll probably require an argument struct for
> layouts, analogous to struct flock for F_SETLK.
> 
> > > > **Unbreakable Layout Leases (F_UNBREAK)**
> > > > 
> > > > In order to support pinning of file pages by direct user space users an
> > > > unbreakable flag (F_UNBREAK) can be used to modify the read and write layout
> > > > lease.  When specified, F_UNBREAK indicates that any user attempting to break
> > > > the lease will fail with ETXTBUSY rather than follow the normal breaking
> > > > procedure.
> > > > 
> > > > Both read and write layout leases can have the unbreakable flag (F_UNBREAK)
> > > > specified.  The difference between an unbreakable read layout lease and an
> > > > unbreakable write layout lease are that an unbreakable read layout lease is
> > > > _not_ exclusive.  This means that once a layout is established on a file,
> > > > multiple unbreakable read layout leases can be taken by multiple processes and
> > > > used to pin the underlying pages of that file.
> > > > 
> > > > Care must therefore be taken to ensure that the layout of the file is as the
> > > > user wants prior to using the unbreakable read layout lease.  A safe mechanism
> > > > to do this would be to take a write layout lease and use fallocate() to set the
> > > > layout of the file.  The layout lease can then be "downgraded" to unbreakable
> > > > read layout as long as no other user broke the write layout lease.
> > > > 
> > > 
> > > Will userland require any special privileges in order to set an
> > > F_UNBREAK lease? This seems like something that could be used for DoS. I
> > > assume that these will never time out.
> > 
> > Dan and I discussed this some more and yes I think the uid of the process needs
> > to be the owner of the file.  I think that is a reasonable mechanism.
> > 
> 
> If that's the model we want to use, then I think the owner of the file
> will need some mechanism to forcibly seize the layout in this event.
> 
> What happens when the file is chowned in that case. Is that also denied?
> If I set an F_UNBREAK layout (maybe as root) and then setuid(), do I get
> to keep the layout?
> 
> > > How will we deal with the case where something is is squatting on an
> > > F_UNBREAK lease and isn't letting it go?
> > 
> > That is a good question.  I had not considered someone taking the UNBREAK
> > without pinning the file.
> > 
> 
> Even if the file is "pinned", I think this is still something that could
> be abused. We need to try to consider how we will address those
> situations up front.
> 
> In that same vein, I know you mentioned that conflicting activity will
> just be denied when there is an outstanding F_UNBREAK lease. Will the
> process holding one be notified in some fashion when another task
> attempts to do some conflicting activity?
> 
> > > Leases are technically "owned" by the file description -- we can't
> > > necessarily trace it back to a single task in a threaded program. The
> > > kernel task that set the lease may have exited by the time we go
> > > looking.
> > > 
> > > Will we be content trying to determine this using /proc/locks+lsof, etc,
> > > or will we need something better?
> > 
> > I think using /proc/locks is our best bet.  Similar to my intention to report
> > files being pinned.[1]
> > 
> > In fact should we consider files with F_UNBREAK leases "pinned" and just report
> > them there?
> > 
> > [1] https://lkml.org/lkml/2019/8/9/1043
> 
> Sure, but eventually you'll want to track that back to a process that is
> holding the thing. /proc/locks just shows you dev+ino for a particular
> lock. You'll need to use something like lsof to figure out who is
> holding the file open.
> 
> Since layouts aren't necessarily broken on open, there may be a bunch of
> tasks that have the file open. How will you identify which one holds the
> F_UNBREAK layout?
> -- 
> Jeff Layton <jlayton@xxxxxxxxxx>