Counter-kludge for 2.5.x hanging when writing to block device

[Adam J. Richter]

	For at least the past few months, the Linux 2.5 kernels have
hung when I try to write a large amount of data to a block device.
I most commonly notice this when trying to clear a disk with a command
like "dd if=/dev/zero of=/dev/discs/disc1/disc".  Sometimes doing
an mkfs on a big file system is enough to cause the hang.
I wrote a little program to repeatedly write a 4kB block of zeroes
to the kernel so I could track how far it got before hanging, and it
would write 210-215MB of zeroes to the disk on a computer that had
512MB of RAM before hanging.  When these hangs occur, other processes
continue to run fine, and I can do syncs, which return, but the
hung process never resumes.  In the past, I've verified with a
printk that it is looping in balance_dirty_pages, repeatedly
calling blk_congestion_wait, and never leaving the loop.

	Here is a counter-kludge that seems to stop the problem.
This is certainly not the "right" fix.  It just illustrates a way
to stop the problem.

	By the way, I say "counter-kludge", because I get the impression
that blk_congestion_wait is itself a kludge, since it calls
blk_run_queues and waits a fixed amount of time, 100ms in this case,
potentially a big waste of time, rather than awaiting some more
accurate criterion.

Adam J. Richter     __     ______________   575 Oroville Road
[e-mail]     \ /                  Miplitas, California 95035
[phone]         | g g d r a s i l   United States of America
                         "Free Software For The Rest Of Us."

--- linux-2.5.70-bk7/mm/page-writeback.c	2003-06-02 14:02:39.000000000 -0700
+++ linux/mm/page-writeback.c	2003-06-02 13:59:31.000000000 -0700
@@ -177,7 +177,12 @@
 			if (pages_written >= write_chunk)
 				break;		/* We've done our duty */
 		}
+#if 0				/* AJR */
 		blk_congestion_wait(WRITE, HZ/10);
+#else
+		blk_run_queues();
+		break;
+#endif
 	}
 
 	if (nr_reclaimable + ps.nr_writeback <= dirty_thresh)