Re: [PATCH v2] btrfs: fix warn_on for send from readonly mount

[Anand Jain <anandsuveer@gmail.com>]

On 12/2/19 11:42 PM, Filipe Manana wrote:
> On Mon, Dec 2, 2019 at 2:26 PM Anand Jain <anand.jain@oracle.com> wrote:
>>
>> We log warning if root::orphan_cleanup_state is not set to
>> ORPHAN_CLEANUP_DONE in btrfs_ioctl_send(). However if the filesystem is
>> mounted as readonly we skip the orphan items cleanup during the lookup
>> and root::orphan_cleanup_state remains at the init state 0 instead of
>> ORPHAN_CLEANUP_DONE (2).
>>
>> WARNING: CPU: 0 PID: 2616 at /Volumes/ws/btrfs-devel/fs/btrfs/send.c:7090 btrfs_ioctl_send+0xb2f/0x18c0 [btrfs]
>> ::
>> RIP: 0010:btrfs_ioctl_send+0xb2f/0x18c0 [btrfs]
>> ::
>> Call Trace:
>> ::
>> _btrfs_ioctl_send+0x7b/0x110 [btrfs]
>> btrfs_ioctl+0x150a/0x2b00 [btrfs]
>> ::
>> do_vfs_ioctl+0xa9/0x620
>> ? __fget+0xac/0xe0
>> ksys_ioctl+0x60/0x90
>> __x64_sys_ioctl+0x16/0x20
>> do_syscall_64+0x49/0x130
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> Reproducer:
>>    mkfs.btrfs -fq /dev/sdb && mount /dev/sdb /btrfs
>>    btrfs subvolume create /btrfs/sv1
>>    btrfs subvolume snapshot -r /btrfs/sv1 /btrfs/ss1
>>    umount /btrfs && mount -o ro /dev/sdb /btrfs
>>    btrfs send /btrfs/ss1 -f /tmp/f
>>
>> Fix this by removing the warn_on completely because:
>>
>> 1) Having orphan items means we could have files to delete (link count
>> of 0) and dealing with such cases could make send fail for several
>> reasons.
>>     If this happens, it's not longer a problem since the following
>> commit:
>>     46b2f4590aab71d31088a265c86026b1e96c9de4
>>     Btrfs: fix send failure when root has deleted files still open
> 
> The convention for mentioning commits is
> first_12_or_slighly_more_hash_characters ("subject").
> scripts/checkpatch.pl warns about it, and given this has been around
> for years, you should already be familiar with it.
> 
>>
>> 2) Orphan items used to indicate previously unfinished truncations, in
>> which case it would lead to send creating corrupt files at the
>> destination (i_size incorrect and the file filled with zeroes between
>> real i_size and stale i_size).
>>     We no longer need to create orphans for truncations since commit:
>>     f7e9e8fc792fe2f823ff7d64d23f4363b3f2203a
>>     Btrfs: stop creating orphan items for truncate
> 
> And I didn't expect you to literally copy-paste what I wrote before.
> For a changelog we want something better written, organized and more
> detailed then an informal e-mail reply, like this:
> 
> "
> The warning exists because having orphanized inodes could confuse send
> and cause it to fail or produce incorrect streams.
> The two cases that would cause problems were:
> 
> 1) Inodes that were unlinked - these are orphanized and remain with a
> link count of 0, having no references (names).
>     These caused send operations to fail because it expected to always
> find at least one path for an inode.
>     This is no longe a problem since send is now able to deal with such
> inodes since
>     commit 46b2f4590aab ("Btrfs: fix send failure when root has deleted
> files still open") and treats them as having
>     been completely removed (the state after a orphan cleanup is performed).
> 
> 2) Inodes that were in the process of being truncated. These resulted
> in send not knowing about the truncation
>      and potentially issue write operations full of zeroes for the
> range from the new file size to the old file size.
>      This is no longer a problem because we no longer create orphan
> items for truncations since
>      commit  f7e9e8fc792f ("Btrfs: stop creating orphan items for truncate")..
> 
> In other words the warning was there to provide a clue in case
> something went wrong. Instead of being a warning
> against the root's "->orphan_cleanup_state" value, it could have been
> more accurate by checking if there were actually
> any orphan items, and then issue a warning only if any exists, but
> that would be more expensive to check.
> Since orphanized inodes no longer cause problems for send, just remove
> the warning.
> "

  Ah. Generally your commits has the best change logs. Its very hard
  to either match or satisfy your level. ;-). But I am trying and not
  there yet.

Thanks. Anand

>>
>> Reported-by: Christoph Anton Mitterer <calestyo@scientia.net>
>> Suggested-by: Filipe Manana <fdmanana@gmail.com>
> 
> Also s/@gmail.com/@suse.com/ (preferable).
> 
> Thanks.
> 
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>> v2:
>>   Remove WARN_ON() completely.
>>
>>   fs/btrfs/send.c | 6 ------
>>   1 file changed, 6 deletions(-)
>>
>> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
>> index ae2db5eb1549..091e5bc8c7ea 100644
>> --- a/fs/btrfs/send.c
>> +++ b/fs/btrfs/send.c
>> @@ -7084,12 +7084,6 @@ long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg)
>>          spin_unlock(&send_root->root_item_lock);
>>
>>          /*
>> -        * This is done when we lookup the root, it should already be complete
>> -        * by the time we get here.
>> -        */
>> -       WARN_ON(send_root->orphan_cleanup_state != ORPHAN_CLEANUP_DONE);
>> -
>> -       /*
>>           * Userspace tools do the checks and warn the user if it's
>>           * not RO.
>>           */
>> --
>> 1.8.3.1
>>
> 
>