dirty balancing deadlock

[Miklos Szeredi]

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.

Ideas:

  - per filesystem dirty counters.  If filesystem is clean (or dirty
    is below some minimum), then balance_dirty_pages() should no wait
    any more

  - throttle_vm_writeout() was meant to throttle swapping, no?  So in
    that case there should be a separate swap-writback counter

Thanks,
Miklos
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