Re: [PATCH 2/2] fat (exportfs): reconnect file handles to evicted inodes/dentries

["Steven J. Magnani" <steve@digidescorp.com>]

On Sat, 2012-07-07 at 15:03 +0900, OGAWA Hirofumi wrote: 
> "Steven J. Magnani" <steve@digidescorp.com> writes:
> 
> > On Sat, 2012-07-07 at 06:07 +0900, OGAWA Hirofumi wrote: 
> >> "Steven J. Magnani" <steve@digidescorp.com> writes:
> >> 
> >> > On Wed, 2012-07-04 at 20:07 +0900, OGAWA Hirofumi wrote: 
> >> >> Please don't add new lock_super() usage if it is not necessary. Almost
> >> >> all of lock_super() just replaced lock_kernel() usage. It rather should
> >> >> be removed in future.  Probably, this should use inode->i_mutex instead.
> >> >> 
> >> >> BTW, the above issue is same with all of directory read.
> >> >
> >> > I don't think there's really an alternative here. The cases addressed by
> >> > this patch all involve walking on-disk structures via
> >> > unofficial/temporary inodes created from information in the NFS handle
> >> > (i.e., outside the normal inode creation paths). When this process is
> >> > successful we end up with "official" connected inodes/dentries, but
> >> > getting there is really a "bottom up" strategy instead of the usual "top
> >> > down" approach.
> >> >
> >> > Because the "bottom up" method is lacking guarantees that "top down"
> >> > takes for granted - i.e., that a cluster on disk that's supposed to be a
> >> > directory actually *is* a directory -  I am adding some defensive code
> >> > in the next spin of the patch.
> >> 
> >> I'm not sure what you meant. Where is the problem? ->get_name()? If so,
> >> it has parent dentry parameter. What is the wrong if we take
> >> mutex_lock(parent->d_inode)?
> >
> > The temporary/"unofficial" inodes are unhashed and thus not visible
> > outside of the functions using them. They exist only to support access
> > to directory contents when we can't gain that access via other means
> > (because we only have "bottom up" information, about an object and
> > perhaps its parent, in a form that can't be used to look up hashed
> > inodes/dentries). Hashing them wouldn't help, because they don't have
> > the correct key (for instance, in the case of a ".." entry).
> >
> > Am I missing something?
> 
> You mean the unhashed inode is created by ->get_parent()? If so, the
> root cause sounds like ->get_parent() itself. If not, I'm not
> understanding the meaning of the temporary/unofficial inode here.

Maybe "private" is a better word than "unofficial". Private inodes are
created anywhere fat_new_inode (nee fat_build_unhashed_inode) is called
directly, instead of through fat_build_inode. So yes, this is on the
get_parent paths (via fat_lookup_dir), and also on the fh_to_dentry path
when inode reconstruction is necessary.

With private inodes, I don't see how anyone but the code that created
them could find them to lock them. The reason they're private is that
they're temporary aliases; at the time they're created, we don't have
enough information to register them in a way that others could find
them. A lookup, etc. operation will look for the inode of the "drivers"
directory, not the ".." of the "usb" directory. We do need these private
inodes in order to walk directory entries. I don't think they're a
problem that needs solving; if we didn't use private inodes, we'd still
need a way to walk directory entries in the context of these NFS
operations, and there would still be potential races between that and
other operations on the filesystem.

------------------------------------------------------------------------
Steven J. Magnani               "I claim this network for MARS!
www.digidescorp.com              Earthling, return my space modulator!"

#include <standard.disclaimer>