Re: [PATCH] mm: Avoid livelocking of WB_SYNC_ALL writeback

[Wu Fengguang <fengguang.wu@intel.com>]

On Sat, Nov 06, 2010 at 06:30:38AM +0800, Christoph Hellwig wrote:
> > +	/*
> > +	 * In WB_SYNC_ALL mode, we just want to ignore nr_to_write as
> > +	 * we need to write everything and livelock avoidance is implemented
> > +	 * differently.
> > +	 */
> > +       if (wbc.sync_mode == WB_SYNC_NONE)
> > +               write_chunk = MAX_WRITEBACK_PAGES;
> > +       else
> > +               write_chunk = LONG_MAX;

Good catch!

> 
> I think it would be useful to elaborate here on how livelock avoidance
> is supposed to work.

It's supposed to sync files in a big loop

        for each dirty inode
            write_cache_pages()
                (quickly) tag currently dirty pages
                (maybe slowly) sync all tagged pages

Ideally the loop should call write_cache_pages() _once_ for each inode.
At least this is the assumption made by commit f446daaea (mm:
implement writeback livelock avoidance using page tagging).

Setting wbc.nr_to_write to LONG_MAX ensures that writeback_inodes_wb()
will complete the above loop before returning to wb_writeback(), and
to prevent wb_writeback() from looping (thus re-syncing extra data) in
the below range of code.

   643                 wbc.nr_to_write = MAX_WRITEBACK_PAGES;
   644                 wbc.pages_skipped = 0;
   645            
   646                 trace_wbc_writeback_start(&wbc, wb->bdi);
   647                 if (work->sb)
   648                         __writeback_inodes_sb(work->sb, wb, &wbc);
   649                 else
   650                         writeback_inodes_wb(wb, &wbc);
   651                 trace_wbc_writeback_written(&wbc, wb->bdi);
   652            
   653                 work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
   654                 wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
   655            
   656                 /*
   657                  * If we consumed everything, see if we have more
   658                  */
   659                 if (wbc.nr_to_write <= 0)
   660                         continue;

Thanks,
Fengguang