Re: [PATCH] writeback: Don't wait for completion inwriteback_inodes_sb_nr

[Jan Kara]

 On Tue 12-07-11 06:41:32, Christoph Hellwig wrote:
> On Tue, Jul 12, 2011 at 12:34:53PM +0200, Jan Kara wrote:
> > > All block device inodes sit on blockdev_superblock, we got rid of inodes
> > > without a superblock long time ago.
> >   Sure, we can easily iterate also blockdev_superblock. What I meant is
> > that blockdev_superblock will need a special handling since we otherwise
> > ignore pseudo superblocks...
> 
> Pseudo superblocks aren't ignored.  They are added to super_blocks like
> all others, and iterate_supers doesn't skip over them.  The problem
> is that blockdev_superblock doesn't have a proper s_bdi set, and thus
> gets skipped over by __sync_filesystem. 
  Yes. But even if it was not skipped writeback_inodes_sb() doesn't have
one flusher thread to kick to actually do the writeout (since each inode on
blockdev_superblock belongs to a different bdi). So it's perfectly fine we
skip blockdev_superblock.

  If we want to fix the problem something like attached patch should do.
Comments?

								Honza

PS: While testing the patch, I've noticed that block device can have any
dirty data only if it is still open (__blkdev_put() writes all dirty pages)
so that somehow limits how much people can be burned by sync not writing
out block devices...
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR