On Mon 11-02-19 09:22:58, Dan Williams wrote: > On Mon, Feb 11, 2019 at 2:24 AM Jan Kara <jack@xxxxxxx> wrote: > > > > On Fri 08-02-19 12:50:37, Dan Williams wrote: > > > On Fri, Feb 8, 2019 at 3:11 AM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > On Fri 08-02-19 15:43:02, Dave Chinner wrote: > > > > > On Thu, Feb 07, 2019 at 04:55:37PM +0000, Christopher Lameter wrote: > > > > > > One approach that may be a clean way to solve this: > > > > > > 3. Filesystems that allow bypass of the page cache (like XFS / DAX) will > > > > > > provide the virtual mapping when the PIN is done and DO NO OPERATIONS > > > > > > on the longterm pinned range until the long term pin is removed. > > > > > > > > > > So, ummm, how do we do block allocation then, which is done on > > > > > demand during writes? > > > > > > > > > > IOWs, this requires the application to set up the file in the > > > > > correct state for the filesystem to lock it down so somebody else > > > > > can write to it. That means the file can't be sparse, it can't be > > > > > preallocated (i.e. can't contain unwritten extents), it must have zeroes > > > > > written to it's full size before being shared because otherwise it > > > > > exposes stale data to the remote client (secure sites are going to > > > > > love that!), they can't be extended, etc. > > > > > > > > > > IOWs, once the file is prepped and leased out for RDMA, it becomes > > > > > an immutable for the purposes of local access. > > > > > > > > > > Which, essentially we can already do. Prep the file, map it > > > > > read/write, mark it immutable, then pin it via the longterm gup > > > > > interface which can do the necessary checks. > > > > > > > > Hum, and what will you do if the immutable file that is target for RDMA > > > > will be a source of reflink? That seems to be currently allowed for > > > > immutable files but RDMA store would be effectively corrupting the data of > > > > the target inode. But we could treat it similarly as swapfiles - those also > > > > have to deal with writes to blocks beyond filesystem control. In fact the > > > > similarity seems to be quite large there. What do you think? > > > > > > This sounds so familiar... > > > > > > https://lwn.net/Articles/726481/ > > > > > > I'm not opposed to trying again, but leases was what crawled out > > > smoking crater when this last proposal was nuked. > > > > Umm, don't think this is that similar to daxctl() discussion. We are not > > speaking about providing any new userspace API for this. > > I thought explicit userspace API was one of the outcomes, i.e. that we > can't depend on this behavior being an implicit side effect of a page > pin? I was thinking an implicit sideeffect of gup_longterm() call. Similarly as swapon(2) does not require the file to be marked in any special way. But OTOH I agree that RDMA is a less controlled usage than swapon so it is questionable. I'd still require something like CAP_LINUX_IMMUTABLE at least for gup_longterm() calls that end up pinning the file. Inspired by Christoph's idea you reference in [2], maybe gup_longterm() will succeed only if there is FL_LAYOUT lease for the range being pinned and we don't allow the lease to be released until there's a pinned page in the range. And we make the file protected (i.e. treat it like swapfile) if there's any such lease in it. But this is just a rough sketch and needs more thinking. > > Also I think the > > situation about leases has somewhat cleared up with this discussion - ODP > > hardware does not need leases since it can use MMU notifiers, for non-ODP > > hardware it is difficult to handle leases as such hardware has only one big > > kill-everything call and using that would effectively mean lot of work on > > the userspace side to resetup everything to make things useful if workable > > at all. > > > > So my proposal would be: > > > > 1) ODP hardward uses gup_fast() like direct IO and uses MMU notifiers to do > > its teardown when fs needs it. > > > > 2) Hardware not capable of tearing down pins from MMU notifiers will have > > to use gup_longterm() (we may actually rename it to a more suitable name). > > FS may just refuse such calls (for normal page cache backed file, it will > > just return success but for DAX file it will do sanity checks whether the > > file is fully allocated etc. like we currently do for swapfiles) but if > > gup_longterm() returns success, it will provide the same guarantees as for > > swapfiles. So the only thing that we need is some call from gup_longterm() > > to a filesystem callback to tell it - this file is going to be used by a > > third party as an IO buffer, don't touch it. And we can (and should) > > probably refactor the handling to be shared between swapfiles and > > gup_longterm(). > > Yes, lets pursue this. At the risk of "arguing past 'yes'" this is a > solution I thought we dax folks walked away from in the original > MAP_DIRECT discussion [1]. Here is where leases were the response to > MAP_DIRECT [2]. ...and here is where we had tame discussions about > implications of notifying memory-registrations of lease break events > [3]. Yeah, thanks for the references. > I honestly don't like the idea that random subsystems can pin down > file blocks as a side effect of gup on the result of mmap. Recall that > it's not just RDMA that wants this guarantee. It seems safer to have > the file be in an explicit block-allocation-immutable-mode so that the > fallocate man page can describe this error case. Otherwise how would > you describe the scenarios under which FALLOC_FL_PUNCH_HOLE fails? So with requiring lease for gup_longterm() to succeed (and the FALLOC_FL_PUNCH_HOLE failure being keyed from the existence of such lease), does it look more reasonable to you? > [1]: https://lwn.net/Articles/736333/ > [2]: https://www.mail-archive.com/linux-nvdimm@xxxxxxxxxxxx/msg06437.html > [3]: https://www.mail-archive.com/linux-nvdimm@xxxxxxxxxxxx/msg06499.html Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
