On Sun, 18 Feb 2007 23:50:14 +0100 Miklos Szeredi <[email protected]> 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?
>
> - filesystems A and B
> - write to A will end up as write to B
> - dirty pages in A manage to go over dirty_threshold
> - page writeback is started from A
> - this triggers writeback for a couple of pages in B
> - writeback finishes normally, but dirty+writeback pages are still
> over threshold
> - balance_dirty_pages in B gets stuck, nothing ever moves after this
>
> At least this is my theory for what happens.
>
Is B a real filesystem? If so, writes to B will decrease the dirty memory
threshold.
The writeout code _should_ just sit there transferring dirtyiness from A to
B and cleaning pages via B, looping around, alternating between both.
What does sysrq-t say?
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