Re: dirty balancing deadlock

From: Rik van Riel
Date: Sun Feb 18 2007 - 16:26:02 EST

Andrew Morton wrote:
On Sun, 18 Feb 2007 19:28:18 +0100 Miklos Szeredi <miklos@xxxxxxxxxx> 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

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.

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