On Fri, Jul 07 2017, Trond Myklebust wrote: > On Fri, 2017-07-07 at 12:41 +1000, NeilBrown wrote: >> On Fri, Jun 30 2017, Trond Myklebust wrote: >> >> > On Thu, 2017-06-29 at 11:46 -0400, J. Bruce Fields wrote: >> > > On Thu, Jun 29, 2017 at 06:34:49AM -0700, Christoph Hellwig >> > > wrote: >> > > > this resurrects parts of an old series to add open by handle >> > > > support to >> > > > NFS. The prime intent here is to support the actual open by >> > > > handle >> > > > ioctls, although it will also allow very crude re- >> > > > exporting. Without >> > > > the other patches from Jeff's series that re-exporting will >> > > > suck >> > > > badly >> > > > though. >> > > >> > > Why do we want this? >> > > >> > > Any re-export support is going to have some major >> > > limitations. (No >> > > file >> > > locking, and re-export of NFSv4 probably not possible?) >> > > >> > > Last I heard the only motivation was extremely specific to >> > > Primary >> > > Data's setup. I'm happy to help them, but I think we need *some* >> > > evidence this will be useful to upstream users. >> > > >> > >> > The main use case for open by filehandle was (and still should be) >> > the >> > promise of being able to do the sort of tricks you normally >> > associate >> > with object storage on a standard filesystem. >> > >> > Imagine that you are trying to build an application for indexing >> > and >> > searching the data on your storage. You basically want to trawl >> > through >> > the filesystem on a regular basis and build up a database of key >> > words >> > and other metadata to tell you what is in the files. For that kind >> > of >> > application, the namespace is a real PITA to deal with, because >> > files >> > get renamed, moved and deleted all the time; so if you can store >> > something that is independent of the namespace and that will give >> > you >> > access to the file contents, then why wouldn't you do so? Normally, >> > applications like that use the inode number, but you can't open a >> > file >> > by inode number, and you have the same problems with inode number >> > reuse >> > that a NFS server has. >> > >> > That's the sort of thing I'd think we want to allow through open by >> > filehandle, and I see no reason why NFS should be excluded from >> > that >> > type of application. >> >> Given that the goal, and presumably the testing, is focused on this >> use-case, I wonder if we should take steps to disable the NFS-re- >> export >> use case. >> As the patch stands, I suspect that NFS re-export would appear to >> work, >> but - as Bruce suggests - would likely hit some problems. This might >> not be a user-friendly thing to do. >> >> Probably the ideal would be to keep re-export disabled by default, >> but >> to allow it to be enabled using a module parameter. >> I'm not sure the best way for NFS to tell nfsd that export shouldn't >> be >> trusted. >> Maybe add a "flags" field to struct export_operations, which can >> contain >> a "No NFS export" flag ?? >> > > You could, but the reason why we developed the code in the first place, > was because we have an internal use case which does involve re-export > of NFS, so we're familiar with the limitations. > > FYI, the limitations are: > 1) Recovery of locks is not possible when the re-exporting server is > the one being rebooted. It works just fine for all other cases. > 2) Re-exporting anything to NFSv2 is not possible. > 3) Re-export of filesystems with very large filenandles could be > problematic. This would also affect open-by-filehandle. In practice it > turns out to be a non-issue because nobody uses filehandles > 64 bytes. > 4) Stateless NFSv3 reads and writes can be a problem with NFSv4 when > the application usees odd modebit settings such as 000. > > IOW: in practice this is no worse than all the other re-exporters such > as already exist for gluster -> NFSv3, ceph -> NFSv3, NFS -> SMB,... > and which everyone seems happy to use. > Thanks. That isn't as bad as I feared. Maybe it would be useful to put notes like in this in the nfs(5) man page. For the NFSv2 export I suspect that in some cases you might be able to successfully mount, but then not access anything. The linux NFS server uses a very small file handles for an export-point. It might be good to have nfs_encode_fh always fail if *max_len <= 32/4 just to ensure that NFSv2 doesn't even get off the ground. Thanks, NeilBrown
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