Re: Writeback bug causing writeback stalls

[Jan Kara]

On Fri 22-05-20 17:23:30, Martijn Coenen wrote:
> [ dropped android-storage-core@xxxxxxxxxx from CC: since that list
> can't receive emails from outside google.com - sorry about that ]
> 
> Hi Jan,
> 
> On Fri, May 22, 2020 at 4:41 PM Jan Kara <jack@xxxxxxx> wrote:
> > > The easiest way to fix this, I think, is to call requeue_inode() at the end of
> > > writeback_single_inode(), much like it is called from writeback_sb_inodes().
> > > However, requeue_inode() has the following ominous warning:
> > >
> > > /*
> > >  * Find proper writeback list for the inode depending on its current state and
> > >  * possibly also change of its state while we were doing writeback.  Here we
> > >  * handle things such as livelock prevention or fairness of writeback among
> > >  * inodes. This function can be called only by flusher thread - noone else
> > >  * processes all inodes in writeback lists and requeueing inodes behind flusher
> > >  * thread's back can have unexpected consequences.
> > >  */
> > >
> > > Obviously this is very critical code both from a correctness and a performance
> > > point of view, so I wanted to run this by the maintainers and folks who have
> > > contributed to this code first.
> >
> > Sadly, the fix won't be so easy. The main problem with calling
> > requeue_inode() from writeback_single_inode() is that if there's parallel
> > sync(2) call, inode->i_io_list is used to track all inodes that need writing
> > before sync(2) can complete. So requeueing inodes in parallel while sync(2)
> > runs can result in breaking data integrity guarantees of it.
> 
> Ah, makes sense.
> 
> > But I agree
> > we need to find some mechanism to safely move inode to appropriate dirty
> > list reasonably quickly.
> >
> > Probably I'd add an inode state flag telling that inode is queued for
> > writeback by flush worker and we won't touch dirty lists in that case,
> > otherwise we are safe to update current writeback list as needed. I'll work
> > on fixing this as when I was reading the code I've noticed there are other
> > quirks in the code as well. Thanks for the report!
> 
> Thanks! While looking at the code I also saw some other paths that
> appeared to be racy, though I haven't worked them out in detail to
> confirm that - the locking around the inode and writeback lists is
> tricky. What's the best way to follow up on those? Happy to post them
> to this same thread after I spend a bit more time looking at the code.

Sure, if you are aware some some other problems, just write them to this
thread. FWIW stuff that I've found so far:

1) __I_DIRTY_TIME_EXPIRED setting in move_expired_inodes() can get lost as
there are other places doing RMW modifications of inode->i_state.

2) sync(2) is prone to livelocks as when we queue inodes from b_dirty_time
list, we don't take dirtied_when into account (and that's the only thing
that makes sure aggressive dirtier cannot livelock sync).

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR