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

[Filipe Manana <fdmanana@gmail.com>]

On Mon, Dec 2, 2019 at 11:59 PM Anand Jain <anandsuveer@gmail.com> wrote:
>
> 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.

Thank you, you flatter me.

If you send a new version, a link tag could also be added:

Link: https://lore.kernel.org/linux-btrfs/21cb5e8d059f6e1496a903fa7bfc0a297e2f5370.camel@scientia.net/

thanks

>
> 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
> >>
> >
> >
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”