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SubjectRe: [PATCH 4.4 038/268] Btrfs: fix scrub to repair raid6 corruption
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On Mon, 2018-05-28 at 12:00 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Liu Bo <bo.li.liu@oracle.com>
>
> [ Upstream commit 762221f095e3932669093466aaf4b85ed9ad2ac1 ]

The diff here is actually from commit 8810f7517a3b ("Btrfs: make raid6
rebuild retry more", mentioned in this commit message). (Sasha, please
try to work out why commit messages and descriptions are getting mixed
up in your auto-selections.)

Maybe stable branches should get the real commit 762221f095e3 as well?

Ben.

> The raid6 corruption is that,
> suppose that all disks can be read without problems and if the content
> that was read out doesn't match its checksum, currently for raid6
> btrfs at most retries twice,
>
> - the 1st retry is to rebuild with all other stripes, it'll eventually
>   be a raid5 xor rebuild,
> - if the 1st fails, the 2nd retry will deliberately fail parity p so
>   that it will do raid6 style rebuild,
>
> however, the chances are that another non-parity stripe content also
> has something corrupted, so that the above retries are not able to
> return correct content.
>
> We've fixed normal reads to rebuild raid6 correctly with more retries
> in Patch "Btrfs: make raid6 rebuild retry more"[1], this is to fix
> scrub to do the exactly same rebuild process.
>
> [1]: https://patchwork.kernel.org/patch/10091755/
>
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> Signed-off-by: David Sterba <dsterba@suse.com>
> Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  fs/btrfs/raid56.c  |   18 ++++++++++++++----
>  fs/btrfs/volumes.c |    9 ++++++++-
>  2 files changed, 22 insertions(+), 5 deletions(-)
>
> --- a/fs/btrfs/raid56.c
> +++ b/fs/btrfs/raid56.c
> @@ -2160,11 +2160,21 @@ int raid56_parity_recover(struct btrfs_r
>   }
>  
>   /*
> -  * reconstruct from the q stripe if they are
> -  * asking for mirror 3
> +  * Loop retry:
> +  * for 'mirror == 2', reconstruct from all other stripes.
> +  * for 'mirror_num > 2', select a stripe to fail on every retry.
>    */
> - if (mirror_num == 3)
> - rbio->failb = rbio->real_stripes - 2;
> + if (mirror_num > 2) {
> + /*
> +  * 'mirror == 3' is to fail the p stripe and
> +  * reconstruct from the q stripe.  'mirror > 3' is to
> +  * fail a data stripe and reconstruct from p+q stripe.
> +  */
> + rbio->failb = rbio->real_stripes - (mirror_num - 1);
> + ASSERT(rbio->failb > 0);
> + if (rbio->failb <= rbio->faila)
> + rbio->failb--;
> + }
>  
>   ret = lock_stripe_add(rbio);
>  
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5056,7 +5056,14 @@ int btrfs_num_copies(struct btrfs_fs_inf
>   else if (map->type & BTRFS_BLOCK_GROUP_RAID5)
>   ret = 2;
>   else if (map->type & BTRFS_BLOCK_GROUP_RAID6)
> - ret = 3;
> + /*
> +  * There could be two corrupted data stripes, we need
> +  * to loop retry in order to rebuild the correct data.
> +  *
> +  * Fail a stripe at a time on every retry except the
> +  * stripe under reconstruction.
> +  */
> + ret = map->num_stripes;
>   else
>   ret = 1;
>   free_extent_map(em);

--
Ben Hutchings, Software Developer   Codethink Ltd
https://www.codethink.co.uk/ Dale House, 35 Dale Street
Manchester, M1 2HF, United Kingdom

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