On Thu, 2 Jan 2014 17:58:33 +0200 Alexander Lyakas <alex.bolshoy@xxxxxxxxx>
wrote:
> Hi Neil,
> I see the following issue: if more than one stripe-head experiences an
> error in raid5_end_write_request, then when first stripe-head is
> handled, it eventually fails the disk, but when the second stripe-head
> is handled, disk is already marked as Faulty, so R5_WriteError is not
> cleared from this second stripe-head, but it still remains in the
> stripe cache.
>
> Below are more details on the flow. I added some prints to display the
> flags values etc.
>
> # raid6 is rebuilding a disk
>
> # disk fails, as a result, raid5_end_write_request is called on two
> stripe-heads:
> [23369]md48066*[raid5_end_write_request:2002] sh[4560].flags=0x101A
> dev[0].flags=0x20003 rdev[dm-6].flags=0x40
> bio=ffff880028744fd8[4592(4560):8] WRITE ERROR
> [13527.467363] [3842]md48066*[raid5_end_write_request:2002]
> sh[4568].flags=0x101B dev[0].flags=0x20003 rdev[dm-6].flags=0x240
> bio=ffff880028742bc0[4600(4568):8] WRITE ERROR
>
> As a result, appropriate r5dev's are marked with R5_WriteError on
> these two stripe-heads.
>
> # raid5 calls analyse_stripe on behalf of stripe-head 4560 ((through
> handle_active_stripes->handle_stripe->analyse_stripe)
>
> # analyse_stripe performs this code:
> if (rdev && test_bit(R5_WriteError, &dev->flags)) {
> /* This flag does not apply to '.replacement'
> * only to .rdev, so make sure to check that*/
> struct md_rdev *rdev2 = rcu_dereference(
> conf->disks[i].rdev);
> if (rdev2 == rdev)
> clear_bit(R5_Insync, &dev->flags);
> if (rdev2 && !test_bit(Faulty, &rdev2->flags)) {
> s->handle_bad_blocks = 1;
> it clears the R5_WriteError and sets handle_bad_blocks on the stripe-head
>
> # handle_stripe performs:
> if (s.handle_bad_blocks)
> for (i = disks; i--; ) {
> struct md_rdev *rdev;
> struct r5dev *dev = &sh->dev[i];
> if (test_and_clear_bit(R5_WriteError, &dev->flags)) {
> /* We own a safe reference to the rdev */
> rdev = conf->disks[i].rdev;
> if (!rdev_set_badblocks(rdev, sh->sector,
> STRIPE_SECTORS, 0))
> md_error(conf->mddev, rdev);
> this marks the rdev as Faulty and fails it (bad blocks are disabled)
>
> # raid5d calls analyse_stripe on stripe-head 4568. However, rdev is
> already marked as Faulty, so it does:
> if (rdev && test_bit(Faulty, &rdev->flags))
> rdev = NULL;
> and later this condition does not hold:
> if (rdev && test_bit(R5_WriteError, &dev->flags)) {
> so R5_WriteError is not cleared, but this stripe-head is still valid
> in the stripe cache.
>
> Dumping the state of this stripe-head in the stripe-cache (through
> custom sysfs entry):
> sh[4568].state=0x1010 disks=5 pdidx=2 qdidx=3
> dev[0].flags=0x20001 rdev[???].flags=0x0 (sector=13688)
> dev[1].flags=0x11 rdev[dm-7].flags=0x2 (sector=13720)
> dev[2].flags=0x11 rdev[dm-8].flags=0x2 (sector=0) *P*
> dev[3].flags=0x11 rdev[dm-9].flags=0x2 (sector=0) *Q*
> dev[4].flags=0x11 rdev[dm-10].flags=0x2 (sector=13656)
>
> # Now user recovers the disk and re-adds the disk back into the array.
> Rebuild starts, and eventually md_do_sync calls sync_request on this
> stripe-head, which calls handle_stripe->analyse_stripe. Now
> analyse_stripe sees the stale R5_WriteError flag and fails the disk,
> which aborts the rebuild. Here is the stack trace, which detects the
> stale R5_WriteError and fails the disk:
> [<ffffffffa02d4543>] error+0x153/0x1d0 [raid456]
> [<ffffffffa0445449>] md_error.part.39+0x19/0xa0 [md_mod]
> [<ffffffffa0445503>] md_error+0x33/0x60 [md_mod]
> [<ffffffffa02da07c>] handle_stripe+0x57c/0x2250 [raid456]
> [<ffffffffa02dcacb>] sync_request+0x18b/0x390 [raid456]
> [<ffffffffa044320c>] md_do_sync+0x76c/0xda0 [md_mod]
> [<ffffffffa044084d>] md_thread+0x10d/0x140 [md_mod]
>
> Does this analysis make sense? I don't know how to fix issue though. I
> am looking at kernel (3.8.13 - as usual), but looking at your
> for-linus branch, it seems like the same would happen.
>
Looks like:
commit 5d8c71f9e5fbdd95650be00294d238e27a363b5c
Author: Adam Kwolek <adam.kwolek@xxxxxxxxx>
Date: Fri Dec 9 14:26:11 2011 +1100
md: raid5 crash during degradation
introduced this problem (in linux 3.3) with an over-simple fix for a problem.
I think that we can not just reverted that patch as
commit 14a75d3e07c784c004b4b44b34af996b8e4ac453
Author: NeilBrown <neilb@xxxxxxx>
Date: Fri Dec 23 10:17:52 2011 +1100
md/raid5: preferentially read from replacement device if possible.
has effected a better fix.
So the patch below should work. Does that make sense to you?
Thanks,
NeilBrown
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index cc055da02e2a..9168173deaf3 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3608,7 +3608,7 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
*/
set_bit(R5_Insync, &dev->flags);
- if (rdev && test_bit(R5_WriteError, &dev->flags)) {
+ if (test_bit(R5_WriteError, &dev->flags)) {
/* This flag does not apply to '.replacement'
* only to .rdev, so make sure to check that*/
struct md_rdev *rdev2 = rcu_dereference(
@@ -3621,7 +3621,7 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
} else
clear_bit(R5_WriteError, &dev->flags);
}
- if (rdev && test_bit(R5_MadeGood, &dev->flags)) {
+ if (test_bit(R5_MadeGood, &dev->flags)) {
/* This flag does not apply to '.replacement'
* only to .rdev, so make sure to check that*/
struct md_rdev *rdev2 = rcu_dereference(
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