On Sat, Aug 10, 2019 at 09:52:31AM +1000, Dave Chinner wrote: > On Fri, Aug 09, 2019 at 03:58:15PM -0700, ira.weiny@xxxxxxxxx wrote: > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > In order to support an opt-in policy for users to allow long term pins > > of FS DAX pages we need to export the LAYOUT lease to user space. > > > > This is the first of 2 new lease flags which must be used to allow a > > long term pin to be made on a file. > > > > After the complete series: > > > > 0) Registrations to Device DAX char devs are not affected > > > > 1) The user has to opt in to allowing page pins on a file with an exclusive > > layout lease. Both exclusive and layout lease flags are user visible now. > > > > 2) page pins will fail if the lease is not active when the file back page is > > encountered. > > > > 3) Any truncate or hole punch operation on a pinned DAX page will fail. > > > > 4) The user has the option of holding the lease or releasing it. If they > > release it no other pin calls will work on the file. > > > > 5) Closing the file is ok. > > > > 6) Unmapping the file is ok > > > > 7) Pins against the files are tracked back to an owning file or an owning mm > > depending on the internal subsystem needs. With RDMA there is an owning > > file which is related to the pined file. > > > > 8) Only RDMA is currently supported > > > > 9) Truncation of pages which are not actively pinned nor covered by a lease > > will succeed. > > This has nothing to do with layout leases or what they provide > access arbitration over. Layout leases have _nothing_ to do with > page pinning or RDMA - they arbitrate behaviour the file offset -> > physical block device mapping within the filesystem and the > behaviour that will occur when a specific lease is held. > > The commit descripting needs to describe what F_LAYOUT actually > protects, when they'll get broken, etc, not how RDMA is going to use > it. Ok yes I've been lax in mixing the cover letter for the series and this first commit message. My apologies. > > > @@ -2022,8 +2030,26 @@ static int do_fcntl_add_lease(unsigned int fd, struct file *filp, long arg) > > struct file_lock *fl; > > struct fasync_struct *new; > > int error; > > + unsigned int flags = 0; > > + > > + /* > > + * NOTE on F_LAYOUT lease > > + * > > + * LAYOUT lease types are taken on files which the user knows that > > + * they will be pinning in memory for some indeterminate amount of > > + * time. > > Indeed, layout leases have nothing to do with pinning of memory. Yep, Fair enough. I'll rework the comment. > That's something an application taht uses layout leases might do, > but it largely irrelevant to the functionality layout leases > provide. What needs to be done here is explain what the layout lease > API actually guarantees w.r.t. the physical file layout, not what > some application is going to do with a lease. e.g. > > The layout lease F_RDLCK guarantees that the holder will be > notified that the physical file layout is about to be > changed, and that it needs to release any resources it has > over the range of this lease, drop the lease and then > request it again to wait for the kernel to finish whatever > it is doing on that range. > > The layout lease F_RDLCK also allows the holder to modify > the physical layout of the file. If an operation from the > lease holder occurs that would modify the layout, that lease > holder does not get notification that a change will occur, > but it will block until all other F_RDLCK leases have been > released by their holders before going ahead. > > If there is a F_WRLCK lease held on the file, then a F_RDLCK > holder will fail any operation that may modify the physical > layout of the file. F_WRLCK provides exclusive physical > modification access to the holder, guaranteeing nothing else > will change the layout of the file while it holds the lease. > > The F_WRLCK holder can change the physical layout of the > file if it so desires, this will block while F_RDLCK holders > are notified and release their leases before the > modification will take place. > > We need to define the semantics we expose to userspace first..... Agreed. I believe I have implemented the semantics you describe above. Do I have your permission to use your verbiage as part of reworking the comment and commit message? Thanks, Ira > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx >
