Messages in this thread | | | Subject | dirty balancing deadlock | From | Miklos Szeredi <> | Date | Sun, 18 Feb 2007 19:28:18 +0100 |
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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
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