Re: 2.6.23-rc6-mm1 -- mkfs stuck in 'D'

[Hugh Dickins <hugh@veritas.com>]

On Wed, 19 Sep 2007, Andy Whitcroft wrote:
> Seems I have a case of a largish i386 NUMA (NUMA-Q) which has a mkfs
> stuck in a 'D' wait:
> 
>  =======================
> mkfs.ext2     D c10220f4     0  6233   6222
>  [<c12194da>] io_schedule_timeout+0x1e/0x28
>  [<c10454b4>] congestion_wait+0x62/0x7a
>  [<c10402af>] get_dirty_limits+0x16a/0x172
>  [<c104040b>] balance_dirty_pages+0x154/0x1be
>  [<c103bda3>] generic_perform_write+0x168/0x18a
>  [<c103be38>] generic_file_buffered_write+0x73/0x107
>  [<c103c346>] __generic_file_aio_write_nolock+0x47a/0x4a5
>  [<c103c3b9>] generic_file_aio_write_nolock+0x48/0x9b
>  [<c105d2d6>] do_sync_write+0xbf/0xfc
>  [<c105d3a0>] vfs_write+0x8d/0x108
>  [<c105d4c3>] sys_write+0x41/0x67
>  [<c100260a>] syscall_call+0x7/0xb
>  =======================

[edited out some bogus lines from stale stack]

> This machine and others have run numerous test runs on this kernel and
> this is the first time I've see a hang like this.

I've been seeing something like that on 4-way PPC64: in my case I've
shells hanging in D state trying to append to kernel build log on ext3
(the builds themselves going on elsewhere, in tmpfs): one of the shells
holding i_mutex and stuck doing congestion_waits from balance_dirty_pages.

> I wonder if this is the ultimate cause of the couple of mainline hangs
> which were seen, but not diagnosed.

My *guess* is that this is peculiar to 2.6.23-rc6-mm1, and from Peter's
mm-per-device-dirty-threshold.patch.  printks showed bdi_nr_reclaimable
0, bdi_nr_writeback 24, bdi_thresh 1 in balance_dirty_pages (though I've
not done enough to check if those really correlate with the hangs),
and I'm wondering if the bdi_stat_sum business is needed on the
!nr_reclaimable path.

So I'm running now with the patch below, good so far, but can't judge
until tomorrow whether it has actually addressed the problem seen.

Not-yet-Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
 mm/page-writeback.c |   53 +++++++++++++++++++-----------------------
 1 file changed, 24 insertions(+), 29 deletions(-)

--- 2.6.23-rc6-mm1/mm/page-writeback.c	2007-09-18 12:28:25.000000000 +0100
+++ linux/mm/page-writeback.c	2007-09-19 20:00:46.000000000 +0100
@@ -379,7 +379,7 @@ static void balance_dirty_pages(struct a
 		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
 		bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
 		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
-				break;
+			break;
 
 		if (!bdi->dirty_exceeded)
 			bdi->dirty_exceeded = 1;
@@ -392,39 +392,34 @@ static void balance_dirty_pages(struct a
 		 */
 		if (bdi_nr_reclaimable) {
 			writeback_inodes(&wbc);
-
+			pages_written += write_chunk - wbc.nr_to_write;
 			get_dirty_limits(&background_thresh, &dirty_thresh,
 				       &bdi_thresh, bdi);
+		}
 
-			/*
-			 * In order to avoid the stacked BDI deadlock we need
-			 * to ensure we accurately count the 'dirty' pages when
-			 * the threshold is low.
-			 *
-			 * Otherwise it would be possible to get thresh+n pages
-			 * reported dirty, even though there are thresh-m pages
-			 * actually dirty; with m+n sitting in the percpu
-			 * deltas.
-			 */
-			if (bdi_thresh < 2*bdi_stat_error(bdi)) {
-				bdi_nr_reclaimable =
-					bdi_stat_sum(bdi, BDI_RECLAIMABLE);
-				bdi_nr_writeback =
-					bdi_stat_sum(bdi, BDI_WRITEBACK);
-			} else {
-				bdi_nr_reclaimable =
-					bdi_stat(bdi, BDI_RECLAIMABLE);
-				bdi_nr_writeback =
-					bdi_stat(bdi, BDI_WRITEBACK);
-			}
+		/*
+		 * In order to avoid the stacked BDI deadlock we need
+		 * to ensure we accurately count the 'dirty' pages when
+		 * the threshold is low.
+		 *
+		 * Otherwise it would be possible to get thresh+n pages
+		 * reported dirty, even though there are thresh-m pages
+		 * actually dirty; with m+n sitting in the percpu
+		 * deltas.
+		 */
+		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
+			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
+			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
+		} else if (bdi_nr_reclaimable) {
+			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
+			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+		}
 
-			if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
-				break;
+		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
+			break;
+		if (pages_written >= write_chunk)
+			break;		/* We've done our duty */
 
-			pages_written += write_chunk - wbc.nr_to_write;
-			if (pages_written >= write_chunk)
-				break;		/* We've done our duty */
-		}
 		congestion_wait(WRITE, HZ/10);
 	}
 
-
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