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>
