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SubjectRe: Bugfix in dquot_transfer()


On Thu, 14 Sep 2000, Jan Kara wrote:

> Yes I agree that if notify_change() blocks we still can account imprecisely.
> I think I didn't understand your proposal. The pointers to structures where
> quota should be charged are already in inode. And if we count current number

Sorry, it was a brainfart ;-/

> of blocks after notify_change() once more all the quota will be counted
> properly. The only problem is that quota can be exceeded this way. We have to check

Nope. You've just shifted the race window (and inverted the
effect) - think what happens if you've got new allocations after the UID
change but before the return from notify_change().

> exceeding before notify_change() because later there is no way to undo what
> notify_change() did.

> Currently I'm thinking about change which would make sence to me (at least at
> the first sight): notify_change() will call dquot_transfer() (currently
> dquot_transfer() calls notify_change()).

Umm... I don't think that it will help anything.

How about the following:
* dquot_{alloc,free}_block() _never_ blocks.
* we have 3 inlined helper functions - alloc_block(), free_block() and
change_xid(). They get exclusion (BKL, spinlock, whatever) and update both
quota and i_blocks.
Consequences:
* quota for filesystems without ->i_blocks is history. It doesn't
work anyway - quota for minixfs is so easy to screw that it's not even
funny.
* we can't print any messages from the dquot_{alloc,free}_block().
Let the helper thread do it - we would just add a request to queue and let
it pick the thing. BTW, use of global buffer for creating the messages is
extremely bad idea - TTY output can block and you've got no protection
around print_warning().
* we have to be careful in {read,write}_dquot(). Frankly, I would
prefer to use the pagecache for quota file rather than messing with
->read() and ->write(). Then we can get an exclusion between updating
dquot and copying it to/from page without blocking. Incidentially, we kill
the set_fs() crap that way.
BTW, changing ->dq_op looks nasty - AFAICS you can easily oops on
access to the methods, since the thing may become NULL between the check
and dereferencing.

Comments?

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