On Fri, 2008-05-02 at 11:38 +1000, Neil Brown wrote: > On Thursday May 1, dwmw2@xxxxxxxxxxxxx wrote: > > On Thu, 2008-05-01 at 16:48 -0400, Christoph Hellwig wrote: > > > Yes, and get_fsid would be extremly useful, especially for those > > > filesystems that already have an uuid in the superblock > > > (*cough*, XFS, *cough*), but it'll need some co-operations with > > > nfs-utils on when to use it. > > > > Why do you need to co-operate with userspace? Userspace shouldn't need > > to do anything -- we'll just generate a suitable fsid/uuid for > > ourselves, unless userspace deliberately overrides it for the export in > > question. > > Actually it is the kernel that doesn't need to do anything.... > Mapping between the filesystem and the filesystem part of the > filehandle is done entirely in user space. > The kernel says "Here is a filehandle fragement, what filesystem > should I be accessing". > > So what you really want is to teach nfs-utils to recognise JFFS2 and > extract an appropriate uuid. > It already uses libblkid to get uuids for ext3 and XFS and others. > Extending that to handle JFFS2 should be much of a drama. For JFFS2, there is no UUID; only i_sb->s_dev. Actually. if we just set FS_REQUIRES_DEV then it would work out OK -- at least for the NFS export. We don't do that though, because it gives behaviour we don't want in other situations, > Why is there a deadlock here? Many file systems have their own locking, and lookup() can end up trying to re-take a lock which readdir() is already holding. In the JFFS2 case, it's the fs-internal inode mutex, which is required because the garbage collector can't use i_mutex for lock ordering reasons. See also the readdir implementation and surrounding comments in fs/xfs/linux-2.6/xfs_file.c -- and the way GFS2 uses gfs2_glock_is_locked_by_me() to avoid the deadlock. The annoying thing is that JFFS2 doesn't even _implement_ i_generation, so you get no more useful information out of the lookup() call anyway :) > Both readdir and lookup are called with i_mutex held on the directory > so there should need to do any extra locking (he said, naively). In > the readdirplus cases, i_mutex is held across both the readdir and the > lookup.... > > One problem with your proposed solution is that filehandles aren't all > the same length, so you cannot reliably leave space for them. Not without moving stuff around during the postprocessing, I suppose. Which isn't very pretty -- but it's prettier than some of the hacks we have at the moment to avoid the deadlock. -- dwmw2 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
