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Date: Thu, 17 Sep 2015 10:31:54 -0800
From: Kent Overstreet <kent.overstreet@...il.com>
To: Denis Bychkov <manover@...il.com>
Cc: Vojtech Pavlik <vojtech@...e.com>, linux-kernel@...r.kernel.org,
linux-bcache@...r.kernel.org,
Emmanuel Florac <eflorac@...ellique.com>,
Jiri Kosina <jkosina@...e.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] bcache: Fix writeback_thread never writing back
incomplete stripes.
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@...il.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
--
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