On Fri, Feb 14, 2014 at 09:45:24PM -0500, J. Bruce Fields wrote: > On Fri, Feb 14, 2014 at 05:40:55PM -0800, Eric W. Biederman wrote: > > "J. Bruce Fields" <bfie...@fieldses.org> writes: > > > > > On Fri, Feb 14, 2014 at 01:43:48PM -0500, Josef Bacik wrote: > > >> A user was running into errors from an NFS export of a subvolume that > > >> had a > > >> default subvol set. When we mount a default subvol we will use > > >> d_obtain_alias() > > >> to find an existing dentry for the subvolume in the case that the root > > >> subvol > > >> has already been mounted, or a dummy one is allocated in the case that > > >> the root > > >> subvol has not already been mounted. This allows us to connect the > > >> dentry later > > >> on if we wander into the path. However if we don't ever wander into the > > >> path we > > >> will keep DCACHE_DISCONNECTED set for a long time, which angers NFS. It > > >> doesn't > > >> appear to cause any problems but it is annoying nonetheless, so simply > > >> unset > > >> DCACHE_DISCONNECTED in the get_default_root case and switch > > >> btrfs_lookup() to > > >> use d_materialise_unique() instead which will make everything play nicely > > >> together and reconnect stuff if we wander into the defaul subvol path > > >> from a > > >> different way. With this patch I'm no longer getting the NFS errors when > > >> exporting a volume that has been mounted with a default subvol set. > > >> Thanks, > > > > > > Looks obviously correct, but based on a quick grep, there are four > > > d_obtain_alias callers outside export methods: > > > > > > - btrfs/super.c:get_default_root() > > > - fs/ceph/super.c:open_root_dentry() > > > - fs/nfs/getroot.c:nfs_get_root() > > > - fs/nilfs2/super.c:nilfs_get_root_dentry() > > > > > > It'd be nice to give them a common d_obtain_alias variant instead of > > > making them all clear this by hand. > > > > I am in favor of one small fix at a time, so that progress is made and > > fixing something just for btrfs seems reasonable for the short term. > > > > > Of those nilfs2 also uses d_splice_alias. I think that problem would > > > best be solved by fixing d_splice_alias not to require a > > > DCACHE_DISCONNECTED dentry; IS_ROOT() on its own should be fine. > > > > You mean by renaming d_splice_alias d_materialise_unique? > > > > Or is there a useful distinction you see that should be preserved > > between the two methods? > > > > Right now my inclination is that everyone should just use > > d_materialise_unique and we should kill d_splice_alias. > > Probably. One remaining distinction: > > - In the local filesystem case if you discover a directory is > already aliased elsewhere, you have a corrupted filesystem and > want to error out the lookup. (Didn't you propose a patch to > do something like that before?) > - In the distributed filesystem this is perfectly normal and we > want to do our best to fix up our local cache to represent > remote reality.
The following keeps the d_splice_alias/d_materialise_unique distinction and (hopefully) fixes Josef's bug, and does a little cleanup (including your suggested DISCONNECTED->CONNECTING rename). Any better idea for the naming of d_materialise_unique? I also didn't try to merge the implementations--the merged d_splice_alias/d_materialise_unique was a little uglier than I expected. I'll keep messing around with it though. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
