Messages in this thread |  | | Date | Thu, 14 Sep 2000 16:30:59 +0200 | From | Jan Kara <> | Subject | Re: Bugfix in dquot_transfer() |
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> > 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(). Quota will be still acounted to old user - it's independent of i_uid contents - so when we switch pointers to dquot structures and move allocated space without blocking there shouldn't be problem...
> > 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? I'll think about it... Now I have to go... Honza
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