Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate_lock

Jan Kara
2021-05-17T11:21:19Z

On Fri 14-05-21 09:17:30, Darrick J. Wong wrote:
> On Fri, May 14, 2021 at 09:19:45AM +1000, Dave Chinner wrote:
> > We've been down this path before more than a decade ago when the
> > powers that be decreed that inode locking order is to be "by
> > structure address" rather than inode number, because "inode number
> > is not unique across multiple superblocks".
> >
> > I'm not sure that there is anywhere that locks multiple inodes
> > across different superblocks, but here we are again....
>
> Hm. Are there situations where one would want to lock multiple
> /mappings/ across different superblocks? The remapping code doesn't
> allow cross-super operations, so ... pipes and splice, maybe? I don't
> remember that code well enough to say for sure.

Splice and friends work one file at a time. I.e., first they fill a pipe
from the file with ->read_iter, then they flush the pipe to the target file
with ->write_iter. So file locking doesn't get coupled there.

> I've been operating under the assumption that as long as one takes all
> the same class of lock at the same time (e.g. all the IOLOCKs, then all
> the MMAPLOCKs, then all the ILOCKs, like reflink does) that the
> incongruency in locking order rules within a class shouldn't be a
> problem.

That's my understanding as well.

> > > It might simply be time to convert all
> > > three XFS inode locks to use the same ordering rules.
> >
> > Careful, there lie dragons along that path because of things like
> > how the inode cluster buffer operations work - they all assume
> > ascending inode number traversal within and across inode cluster
> > buffers and hence we do have locking order constraints based on
> > inode number...
>
> Fair enough, I'll leave the ILOCK alone. :)

OK, so should I change the order for invalidate_lock or shall we just leave
that alone as it is not a practical problem AFAICT.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR