Re: [PATCH 07/15] btrfs: Move FS error state bit early during write

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On 12:10 23/09, Nikolay Borisov wrote:
> 
> 
> On 21.09.20 г. 17:43 ч., Goldwyn Rodrigues wrote:
> > From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > 
> > fs_info->fs_state is a filesystem bit check as opposed to inode
> > and can be performed before we begin with write checks. This eliminates
> > inode lock/unlock in case of error bit is set.
> > 
> > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > ---
> >  fs/btrfs/file.c | 21 +++++++++------------
> >  1 file changed, 9 insertions(+), 12 deletions(-)
> > 
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index 4c40a2742aab..ca374cb5ffc9 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -1981,6 +1981,15 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
> >  	size_t count;
> >  	loff_t oldsize;
> >  
> > +	/*
> > +	 * If BTRFS flips readonly due to some impossible error
> > +	 * (fs_info->fs_state now has BTRFS_SUPER_FLAG_ERROR),
> > +	 * although we have opened a file as writable, we have
> > +	 * to stop this write operation to ensure FS consistency.
> > +	 */
> > +	if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state))
> > +		return -EROFS;
> > +
> 
> nit: Actually can't this check be eliminated altogether or the comment
> vastly simplified because BTRFS_SUPER_FLAG_ERROR check is performed only
> during mount so the description in the parantheses is invalid i.e the fs
> won't flip to RO because BTRFS_SUPER_FLAG_ERROR is now set in the super
> block. As a matter of fact how is this flag set - because I don't see it
> set in the kernel code nor in btrfs-progs ?

You are right. This flag originated from 
acce952b0263 ("Btrfs: forced readonly mounts on errors")

However, the following commit removed writing the super in case of the
error:
68ce9682a4bb ("Btrfs: remove superblock writing after fatal error")

So, it does not land in the super flags anyways.
The flag does not make sense if we use BTRFS_FS_STATE_ERROR.

I will remove BTRFS_SUPER_FLAG_ERROR comment for now.

-- 
Goldwyn