Re: [PATCH] bcache: Fix writeback_thread never writing back incomplete stripes.

[Kent Overstreet <kent.overstreet@gmail.com>]

On Thu, Sep 17, 2015 at 08:40:54AM -0800, Kent Overstreet wrote:
> On Thu, Sep 17, 2015 at 11:30:17AM -0400, Denis Bychkov wrote:
> > Well, it turns out my celebration was a bit premature.
> > 
> > PLEASE, DO NOT APPLY THE PATCH POSTED BY KENT (not the one Vojtech
> > posted) ON A PRODUCTION SYSTEM, IT CAUSES DATA CORRUPTION.
> > 
> > The interesting thing is that it somehow damaged the partition that
> > was not supposed to receive any writes (the file system was mounted
> > read-only), so my guess is that the patch causes the blocks residing
> > in the write-back cache to flush to the wrong blocks on the backing
> > device.
> > Everything was going great until I rebooted and saw this in the log:
> > 
> > [   19.639082] attempt to access beyond end of device
> > [   19.643984] md1p2: rw=1, want=75497520, limit=62914560
> > [   19.659033] attempt to access beyond end of device
> > [   19.663929] md1p2: rw=1, want=75497624, limit=62914560
> > [   19.669447] attempt to access beyond end of device
> > [   19.674338] md1p2: rw=1, want=75497752, limit=62914560
> > [   19.679195] attempt to access beyond end of device
> > [   19.679199] md1p2: rw=1, want=75498080, limit=62914560
> > [   19.689007] attempt to access beyond end of device
> > [   19.689011] md1p2: rw=1, want=75563376, limit=62914560
> > [   19.699055] attempt to access beyond end of device
> > [   19.699059] md1p2: rw=1, want=79691816, limit=62914560
> > [   19.719246] attempt to access beyond end of device
> > [   19.724144] md1p2: rw=1, want=79691928, limit=62914560
> > ......
> > (it's a small example, the list was much longer)
> > And the next thing I found out the super block on my 10-Tb XFS RAID was gone. :)
> > Oh well, it's a good thing I have backups.
> > I knew what I was doing when trying the untested patches. I should
> > have made the RAID md partition read-only, not the file system. I kind
> > of expected that something could have gone wrong with the file system
> > I was testing, just did not expect it would fire nukes at the innocent
> > bystanders.
> 
> Aw, shit. That's just _bizzare_.
> 
> I have a theory - it appears that last_scanned isn't getting initialized before
> it's used, so it's going to be all 0s the very first time... which it appears
> could cause it to slurp up keys from the wrong device (and if that device was
> bigger than the correct device, that could explain the accesses beyond the end
> of the device).
> 
> Currently just a theory though, and I have no clue why it would only be exposed
> with my patch.

Here's an updated patch that has a fix for _that_ theory, and also a new
BUG_ON(). Any chance you could test it?

Oh - I didn't ask - _do_ you have multiple backing devices attached to the same
cache set? Because if you don't, this isn't it at all...

-- >8 --
Subject: [PATCH] bcache: Change refill_dirty() to always scan entire disk if necessary

Previously, it would only scan the entire disk if it was starting from the very
start of the disk - i.e. if the previous scan got to the end.

This was broken by refill_full_stripes(), which updates last_scanned so that
refill_dirty was never triggering the searched_from_start path.

But if we change refill_dirty() to always scan the entire disk if necessary,
regardless of what last_scanned was, the code gets cleaner and we fix that bug
too.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
---
 drivers/md/bcache/writeback.c | 37 ++++++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index cdde0f32f0..d383024247 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -310,6 +310,10 @@ void bcache_dev_sectors_dirty_add(struct cache_set *c, unsigned inode,
 
 static bool dirty_pred(struct keybuf *buf, struct bkey *k)
 {
+	struct cached_dev *dc = container_of(buf, struct cached_dev, writeback_keys);
+
+	BUG_ON(KEY_INODE(k) != dc->disk.id);
+
 	return KEY_DIRTY(k);
 }
 
@@ -359,11 +363,24 @@ next:
 	}
 }
 
+/*
+ * Returns true if we scanned the entire disk
+ */
 static bool refill_dirty(struct cached_dev *dc)
 {
 	struct keybuf *buf = &dc->writeback_keys;
+	struct bkey start = KEY(dc->disk.id, 0, 0);
 	struct bkey end = KEY(dc->disk.id, MAX_KEY_OFFSET, 0);
-	bool searched_from_start = false;
+	struct bkey start_pos;
+
+	/*
+	 * make sure keybuf pos is inside the range for this disk - at bringup
+	 * we might not be attached yet so this disk's inode nr isn't
+	 * initialized then
+	 */
+	if (bkey_cmp(&buf->last_scanned, &start) < 0 ||
+	    bkey_cmp(&buf->last_scanned, &end) > 0)
+		buf->last_scanned = start;
 
 	if (dc->partial_stripes_expensive) {
 		refill_full_stripes(dc);
@@ -371,14 +388,20 @@ static bool refill_dirty(struct cached_dev *dc)
 			return false;
 	}
 
-	if (bkey_cmp(&buf->last_scanned, &end) >= 0) {
-		buf->last_scanned = KEY(dc->disk.id, 0, 0);
-		searched_from_start = true;
-	}
-
+	start_pos = buf->last_scanned;
 	bch_refill_keybuf(dc->disk.c, buf, &end, dirty_pred);
 
-	return bkey_cmp(&buf->last_scanned, &end) >= 0 && searched_from_start;
+	if (bkey_cmp(&buf->last_scanned, &end) < 0)
+		return false;
+
+	/*
+	 * If we get to the end start scanning again from the beginning, and
+	 * only scan up to where we initially started scanning from:
+	 */
+	buf->last_scanned = start;
+	bch_refill_keybuf(dc->disk.c, buf, &start_pos, dirty_pred);
+
+	return bkey_cmp(&buf->last_scanned, &start_pos) >= 0;
 }
 
 static void bch_writeback(struct cached_dev *dc)
-- 
2.5.1