Re: [RFC PATCH 2/5 v3] Btrfs: avoid transaction stuff when btrfs is readonly

[Chris Mason <chris.mason@oracle.com>]

Excerpts from liubo's message of 2010-12-15 04:12:14 -0500:
> On 12/15/2010 04:45 PM, Yan, Zheng wrote:
> > On Fri, Dec 3, 2010 at 4:16 PM, liubo <liubo2009@cn.fujitsu.com> wrote:
> >> When the filesystem is readonly, avoid transaction stuff by checking MS_RDONLY
> >> at start transaction time.
> >>
> >> Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
> >> ---
> >>  fs/btrfs/transaction.c |    3 +++
> >>  1 files changed, 3 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> >> index 1fffbc0..14a597d 100644
> >> --- a/fs/btrfs/transaction.c
> >> +++ b/fs/btrfs/transaction.c
> >> @@ -181,6 +181,9 @@ static struct btrfs_trans_handle *start_transaction(struct btrfs_root *root,
> >>        struct btrfs_trans_handle *h;
> >>        struct btrfs_transaction *cur_trans;
> >>        int ret;
> >> +
> >> +       if (root->fs_info->sb->s_flags & MS_RDONLY)
> >> +               return ERR_PTR(-EROFS);
> >>  again:
> >>        h = kmem_cache_alloc(btrfs_trans_handle_cachep, GFP_NOFS);
> >>        if (!h)
> > 
> > There are cases that we need to start transaction when MS_RDONLY flag is set.
> > For example, remount FS into read-only mode and log replay.
> 
> However, is it weird to make changes to disk as fs is in readonly state?
> IMO, btrfs needs to limit the use of these "disk-change while readonly" cases,
> as it is not what readonly means.

reiserfs and ext3 at least have always done this.  Log replay is
required even when the FS is readonly.

> 
> Since it has been here, we can bypass readonly in those cases(as I did in the 5th patch):
> 
> ...
> flags = sb->s_flags;
> if (sb->s_flags & MS_RDONLY)
>     sb->s_flags &= ~MS_RDONLY

I think we should have a dedicated flag to reflect a filesystem that is
forced readonly, and check that flag instead.

-chris