Quota 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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Subject: Re: Quota mods needed for journaled quota
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This would allow ext3 to do that which it needs to do differently at Q_QUOTAON and would also allow Jan's changes to work in such a way that both the current form of dquot structure and his new version of dquots could be used together

Adding the init_quota hook would do that, as the filesystem will be able to install its own dq_ops methods during the init so we get the flexibility you are asking for anyway.

Hmmm ... I'm not so sure. In order to have the flexibility of filesystem-specific dquot formats, the struct dquot would need to become more like struct inode/super_block, i.e. not hardcoding the ondisk structure into the incore structure (using a union and a generic pointer, as inode/super_block do).

The DQUOT_SYNC mechanism would need to be able to be overridden per-filesystem also. It isn't really as cut-and-dried as "per- filesystem" either, because an ext2/3 filesystem might make use of either the original dquot format or Jan's newer format, either at mount time or even after doing a quota_off & quota_on with a new quota file format (that would be quite clean).