dirty balancing deadlock

From: Miklos Szeredi
Date: Sun Feb 18 2007 - 13:29:11 EST

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

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.


- 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

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