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SubjectQuota mods needed for journaled quota

There are a few problems in the Linux quota code which make it
impossible to perform quota updates transactionally when using a
journaled filesystem.

Basically we have the following problems:

* The underlying filesystem does not know which files are the quota
files, so cannot tell when to apply journaling consistency
guarantees to data writes

* "chown" is not transactional: the filesystem is given no
opportunity to wrap the quota transfer and the owner-attribute
notify-change call into a single transaction.

All of these could be fixed very easily (at least for ext3) if it were
possible for ext3 to install its own version of the superblock->dq_ops
quota operations (which would just be simple wrappers around the
existing quota calls). However, the current sys_quotactl installs the
default quota_ops into the superblock on quota_on without any chance
for the filesystem to override it.

The addition of an "init_quota" method to the super_operations struct,
with quota_on calling this and defaulting to installing the default
quota_ops if the method is NULL, ought to be sufficient to let ext3
get quotas right in all cases as far as I can see.


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