Re: dirty balancing deadlock

[Miklos Szeredi <miklos@szeredi.hu>]

> Andrew Morton wrote:
> > On Sun, 18 Feb 2007 19:28:18 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:
> > 
> >> I was testing the new fuse shared writable mmap support, and finding
> >> that bash-shared-mapping deadlocks (which isn't so strange ;).  What
> >> is more strange is that this is not an OOM situation at all, with
> >> plenty of free and cached pages.
> >>
> >> A little more investigation shows that a similar deadlock happens
> >> reliably with bash-shared-mapping on a loopback mount, even if only
> >> half the total memory is used.
> >>
> >> The cause is slightly different in the two cases:
> >>
> >>   - loopback mount: allocation by the underlying filesystem is stalled
> >>     on throttle_vm_writeout()
> >>
> >>   - fuse-loop: page dirtying on the underlying filesystem is stalled on
> >>     balance_dirty_pages()
> >>
> >> In both cases the underlying fs is totally innocent, with no
> >> dirty/writback pages, yet it's waiting for the global dirty+writeback
> >> to go below the threshold, which obviously won't, until the
> >> allocation/dirtying succeeds.
> >>
> >> I'm not quite sure what the solution is, and asking for thoughts.
> > 
> > But....  these things don't just throttle.  They also perform large amounts
> > of writeback, which causes the dirty levels to subside.
> > 
> >>From your description it appears that this writeback isn't happening, or
> > isn't working.  How come?
> 
> Is the fuse daemon trying to do writeback to itself, perhaps?
> 
> That is, trying to write out data to the FUSE filesystem, for which
> it is also the server.

No.  It's trying to write out data to a different filesystem.

Trying to write out data to itself very obviously deadlocks, but that
doesn't affect anything beside the stupid filesystem itself, and there
are mechanisms for aborting such a situation (forced umount, abort
through fuse-control filesystem).

Miklos