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SubjectRe: Bugfix in dquot_transfer()
> 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().
Seems I'll have to read that part of kernel... I didn't expect writing
to console can block (especially as this code was there for ages).

> * 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.
I think it's reasonable (I wanted to move quota to pagecache anyway...).

> 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.
True.. I'll think off how to fix it.

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