On Sat, Aug 24, 2024 at 12:02 AM Qu Wenruo <wqu@xxxxxxxx> wrote: > > [BUG] > There are several bug reports of btrfs/125 failure recently, either > causing balance failure (-EIO), or even kernel crash. > > The balance failure looks like this: > > Mount normal and balance > +ERROR: error during balancing '/mnt/scratch': Input/output error > +There may be more info in syslog - try dmesg | tail > +md5sum: /mnt/scratch/tf2: Input/output error It's been like this forever. Historically there are periods where this happens with some frequency, then for no reason it happens very, very, very, sporadically, then after some time it comes back, and so on. This has been pointed out and discussed several times in the past, for example: https://lore.kernel.org/linux-btrfs/CAL3q7H4oa70DUhOFE7kot62KjxcbvvZKxu62VfLpAcmgsinBFw@xxxxxxxxxxxxxx/ https://lore.kernel.org/linux-btrfs/53f7bace2ac75d88ace42dd811d48b7912647301.1654672140.git.wqu@xxxxxxxx/#r It would be good to add links to such discussions in the changelog. > > [CAUSE] > There are several different factors involved. > > 1. RMW mix the old and new metadata, causing unrepairable corruption > E.g. with the following layout: > > data 1 |<- Stale metadata ->| (from the out-of-date device) > data 2 | Unused | > parity |PPPPPPPPPPPPPPPPPPPP| > > In above case, although metadata on data 1 is out-of-date, we can > still rebuild the correct data from parity and data 2. > > But if we have new metadata writes into the data 2 stripe, an RMW > will screw up the whole situation: > > data 1 |<- Stale metadata ->| (from the out-of-date device) > data 2 |<- New metadata ->| > parity |XXXXXXXXXXXXXXXXXXXX| > > The RMW will use the stale metadata and new metadata to calculate new > parity. > The resulted new parity will no longer be able to recover the old > data 1. > > This is a known bug, thus our documentation is already recommending > to avoid RAID56 for metadata usage. > > > Metadata > > Do not use raid5 nor raid6 for metadata. Use raid1 or raid1c3 > > respectively. > > Furthermore this is very hard to fix, unlike data we can fetch the > data csum and verify during RMW, we can not do that during RMW. > > At the timing of RMW, we're holding the rbio lock for the full > stripe. > If the extent tree search requires a read-recover, it will generate > another rbio, which may cover the same full stripe we're working on, > leading to a deadlock. > > Furthermore the current RAID56 repair code is all based on veritical > sectors, but metadata can cross several horizontal sectors. > This will require multiple combinations to repair a metadata. > > 2. Crash caused by double freeing a bio > By chance if the above RMW corrupted csum tree, then during > btrfs_submit_chunk() we will hit an error path that leads to double > freeing of a bio, leading to crash when hitting the race window. > > Thankfully the patch has been sent to the mailing list. If the changelog is mentioning this bug, then it should also mention which kernel patch fixes it. With that update: Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx> Thanks. > > [WORKAROUND] > Since it's very hard to fix the RAID56 metadata problem without a > deadlock or a huge code rework, for now just use RAID1 for the metadata > of this particular test case. > > There may be a chance to fix the situation by properly marking the > missing-then-reappear device as out-of-date, so no direct read from that > device. > > But that will also be a huge new feature, not something can be done > immediately. > > Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> > --- > tests/btrfs/125 | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/tests/btrfs/125 b/tests/btrfs/125 > index 31379d81ef73..c8c0dd422f72 100755 > --- a/tests/btrfs/125 > +++ b/tests/btrfs/125 > @@ -70,7 +70,7 @@ count=$(($max_fs_sz / 1000000)) > echo >> $seqres.full > echo "max_fs_sz=$max_fs_sz count=$count" >> $seqres.full > echo "-----Initialize -----" >> $seqres.full > -_scratch_pool_mkfs "-mraid5 -draid5" >> $seqres.full 2>&1 > +_scratch_pool_mkfs "-mraid1 -draid5" >> $seqres.full 2>&1 > _scratch_mount >> $seqres.full 2>&1 > dd if=/dev/zero of="$SCRATCH_MNT"/tf1 bs=$bs count=1 \ > >>$seqres.full 2>&1 > -- > 2.46.0 > >
