This patch allows a Network Block Device to be mounted locally
(nbd-client to nbd-server over 127.0.0.1).
It creates a kthread to avoid the deadlock described in NBD tools
documentation. So, if nbd-client hangs waiting for pages, the kblockd
thread can continue its work and free pages.
I have tested the patch to verify that it avoids the hang that occurs
when writing to a localhost nbd connection. I have also tested to verify
that no performance degradation results from the additional thread and
Patch originally from Laurent Vivier.
Well, we expect that. The reason for the new thread is so that it hangs,
rather than kblockd hanging (which on a UP system brings all I/O to a
halt). As long as kblockd can continue making progress, we eventually
free up memory and then NBD can finish its requests, too.
Note that I'm not a kernel hacker, so might be terribly mistaken here...
but I feel I should point out that this patch solves the issue that no
two block devices can flush their dirty buffers at the same time.
Without this patch, you can't write to a _filesystem_ on an NBD device
if that's connected to a server on the localhost. You are correct that
this does not solve the deadlock in swapping to NBD devices, but that's
not the only existing deadlock issue in NBD to localhost...
<Lo-lan-do> Home is where you have to wash the dishes.
-- #debian-devel, Freenode, 2004-09-22