Re: dirty balancing deadlock

[Miklos Szeredi <miklos@szeredi.hu>]

> > > > > In general, writepage is supposed to do work without blocking on
> > > > > expensive locks that will get pdflush and dirty reclaim stuck in this
> > > > > fashion.  You'll probably have to take the same approach reiserfs does
> > > > > in data=journal mode, which is leaving the page dirty if fuse_get_req_wp
> > > > > is going to block without making progress.
> > > > 
> > > > Pdflush, and dirty reclaim set wbc->nonblocking to true.
> > > > balance_dirty_pages and fsync don't.  The problem here is that
> > > > Andrew's patch is wrong to let balance_dirty_pages() try to write back
> > > > pages from a different queue.
> > > 
> > > async or sync, writepage is supposed to either make progress or bail.
> > > loopback aside, if the fuse call is blocking long term, you're going to
> > > run into problems.
> > 
> > Hmm, like what?
> 
> Something a little different from what you're seeing.  Basically if the
> PF_MEMALLOC paths end up waiting on a filesystem transaction, and that
> transaction is waiting for more ram, the system will eventually grind to
> a halt.  data=journal is the easiest way to hit it, since writepage
> always logs at least 4k.
> 
> WB_SYNC_NONE and wbc->nonblocking aren't a great test, in reiser I
> resorted to testing PF_MEMALLOC.

I'm not pretending to understand how journaling filesystems work, but
this shouldn't be an issue with fuse.  Can you show me a call path,
where PF_MEMALLOC is set and .nonblocking is not?

Thanks,
Miklos