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
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