On Fri, May 13, 2011 at 01:03:56PM +0800, Wu Fengguang wrote:
> On Fri, May 13, 2011 at 07:04:32AM +0800, Dave Chinner wrote:
> > On Thu, May 12, 2011 at 09:57:19PM +0800, Wu Fengguang wrote:
> > > When wbc.more_io was first introduced, it indicates whether there are
> > > at least one superblock whose s_more_io contains more IO work. Now with
> > > the per-bdi writeback, it can be replaced with a simple b_more_io test.
> > >
> > > Acked-by: Jan Kara <jack@xxxxxxx>
> > > Acked-by: Mel Gorman <mel@xxxxxxxxx>
> > > CC: Minchan Kim <minchan.kim@xxxxxxxxx>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > > ---
> > > fs/fs-writeback.c | 9 ++-------
> > > include/linux/writeback.h | 1 -
> > > include/trace/events/ext4.h | 6 ++----
> > > include/trace/events/writeback.h | 5 +----
> > > 4 files changed, 5 insertions(+), 16 deletions(-)
> > >
> > > --- linux-next.orig/fs/fs-writeback.c 2011-05-05 23:30:30.000000000 +0800
> > > +++ linux-next/fs/fs-writeback.c 2011-05-05 23:30:33.000000000 +0800
> > > @@ -560,12 +560,8 @@ static int writeback_sb_inodes(struct su
> > > iput(inode);
> > > cond_resched();
> > > spin_lock(&wb->list_lock);
> > > - if (wbc->nr_to_write <= 0) {
> > > - wbc->more_io = 1;
> > > + if (wbc->nr_to_write <= 0)
> > > return 1;
> > > - }
> > > - if (!list_empty(&wb->b_more_io))
> > > - wbc->more_io = 1;
> > > }
> > > /* b_io is empty */
> > > return 1;
> > > @@ -707,7 +703,6 @@ static long wb_writeback(struct bdi_writ
> > > wbc.older_than_this = &oldest_jif;
> > > }
> > >
> > > - wbc.more_io = 0;
> > > wbc.nr_to_write = write_chunk;
> > > wbc.pages_skipped = 0;
> > > wbc.inodes_cleaned = 0;
> > > @@ -755,7 +750,7 @@ retry:
> > > /*
> > > * No more inodes for IO, bail
> > > */
> > > - if (!wbc.more_io)
> > > + if (list_empty(&wb->b_more_io))
> > > break;
> >
> > We're not holding the wb->list_lock here, so we need to be careful
> > here. I think this is safe given that there shuold only be one
> > flusher thread operating on the list, but when we expand to multiple
> > flusher threads per-bdi, this coul dbe a nasty landmine. A comment
> > is probably in order explaining why this is safe to check unlocked
> > right now...
>
> OK, how about this?
>
> /*
> * No more inodes for IO, bail. The unlocked check is safe
> * because each &wb will be worked by only one flusher thread.
> */
> if (list_empty(&wb->b_more_io))
> break;
>
> I guess in future multiple flusher threads will be working on
> different bdi_writeback instances, so it will still be safe.
That's making assumptions about something that hasn't been
implemented yet.
> However for now there are possible interactions from the IO-full
> balance_dirty_pages(). So it looks better to just do the tests inside
> the lock:
Agreed, safer that way.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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