Re: [PATCH RFC] btrfs: self heal from SB fail

[Nikolay Borisov <nborisov@suse.com>]

On  8.12.2017 12:33, Anand Jain wrote:
> 
> 
> On 12/08/2017 04:40 PM, Nikolay Borisov wrote:
>>
>>
>> On  8.12.2017 09:57, Anand Jain wrote:
>>> -EXPERIMENTAL-
>>> As of now when primary SB fails we won't self heal and would fail mount,
>>> this is an experimental patch which thinks why not go and read backup
>>> copy.
>>>
>>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>>> ---
>>>   fs/btrfs/disk-io.c |  8 +++++++-
>>>   fs/btrfs/volumes.c | 10 +++++++---
>>>   2 files changed, 14 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>>> index 9b20c1f3563b..a791b8dfe8a8 100644
>>> --- a/fs/btrfs/disk-io.c
>>> +++ b/fs/btrfs/disk-io.c
>>> @@ -3190,7 +3190,7 @@ struct buffer_head *btrfs_read_dev_super(struct
>>> block_device *bdev)
>>>        * So, we need to add a special mount option to scan for
>>>        * later supers, using BTRFS_SUPER_MIRROR_MAX instead
>>>        */
>>> -    for (i = 0; i < 1; i++) {
>>> +    for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
>>>           ret = btrfs_read_dev_one_super(bdev, i, &bh);
>>>           if (ret)
>>>               continue;
>>> @@ -4015,11 +4015,17 @@ static int btrfs_check_super_valid(struct
>>> btrfs_fs_info *fs_info)
>>>           ret = -EINVAL;
>>>       }
>>>   +#if 0
>>> +    /*
>>> +     * Need a way to check for any copy of SB, as its not a
>>> +     * strong check, just ignore this for now.
>>> +     */
>>>       if (btrfs_super_bytenr(sb) != BTRFS_SUPER_INFO_OFFSET) {
>>>           btrfs_err(fs_info, "super offset mismatch %llu != %u",
>>>                 btrfs_super_bytenr(sb), BTRFS_SUPER_INFO_OFFSET);
>>>           ret = -EINVAL;
>>>       }
>>> +#endif
>>>         /*
>>>        * Obvious sys_chunk_array corruptions, it must hold at least
>>> one key
>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>> index 9fa2539a8493..f368db94d62b 100644
>>> --- a/fs/btrfs/volumes.c
>>> +++ b/fs/btrfs/volumes.c
>>> @@ -1369,7 +1369,7 @@ int btrfs_scan_one_device(const char *path,
>>> fmode_t flags, void *holder,
>>>   {
>>>       struct btrfs_super_block *disk_super;
>>>       struct block_device *bdev;
>>> -    struct page *page;
>>> +    struct buffer_head *sb_bh;
>>>       int ret = -EINVAL;
>>>       u64 devid;
>>>       u64 transid;
>>> @@ -1392,8 +1392,12 @@ int btrfs_scan_one_device(const char *path,
>>> fmode_t flags, void *holder,
>>>           goto error;
>>>       }
>>>   -    if (btrfs_read_disk_super(bdev, bytenr, &page, &disk_super))
>>> +    sb_bh = btrfs_read_dev_super(bdev);
>>
>> This patch prompts another question: why do we have a page-based and a
>> bufferhead-based interface to reading the super block?
> 
>  Right. we need to know that. Sorry I just saw this.

FWIW unless we explicitly need a per-block state tracking (which I don't
think we do) we should ideally switch to page-based mechanism. Buffer
heads are considered deprecated. Also the only reason why we do have
btrfsic_submit_bh is for the superblock bufer head. So potentially by
removing the only requirement in the kernel where bh are used we can
simplify the btrfsic code as well . Just something to keep in mind.

> 
>  I have a very old patch to converge them and clean this up, but haven't
>  sent it because there are some missing information on why it ended up
>  like that in the first place.
> 
> Thanks, Anand
> 
> 
>> I did prototype
>> switching the bufferheads to page based but the resulting code wasn't
>> any cleaner. I believe there is also open the question what happens when
>> btrfs is run on a 64k page machine. I.e. we are going to read a single
>> page and the sb is going to be in the first 4k but what about the rest
>> 60, they could potentially contain other metadata. The page will have to
>> be freed asap so as not to peg the neighboring metadata?
> 
> 
>>
>>> +    if (IS_ERR(sb_bh)) {
>>> +        ret = PTR_ERR(sb_bh);
>>>           goto error_bdev_put;
>>> +    }
>>> +    disk_super = (struct btrfs_super_block *) sb_bh->b_data;
>>>         devid = btrfs_stack_device_id(&disk_super->dev_item);
>>>       transid = btrfs_super_generation(disk_super);
>>> @@ -1413,7 +1417,7 @@ int btrfs_scan_one_device(const char *path,
>>> fmode_t flags, void *holder,
>>>       if (!ret && fs_devices_ret)
>>>           (*fs_devices_ret)->total_devices = total_devices;
>>>   -    btrfs_release_disk_super(page);
>>> +    brelse(sb_bh);
>>>     error_bdev_put:
>>>       blkdev_put(bdev, flags);
>>>
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