When wb_writeback() is called in WB_SYNC_ALL mode, work->nr_to_write is usually set to LONG_MAX. The logic in wb_writeback() then calls __writeback_inodes_sb() with nr_to_write == MAX_WRITEBACK_PAGES and thus we easily end up with negative nr_to_write after the function returns. wb_writeback() then decides we need another round of writeback but this is wrong in some cases! For example when a single large file is continuously dirtied, we would never finish syncing it because each pass would be able to write MAX_WRITEBACK_PAGES and inode dirty timestamp never gets updated (as inode is never completely clean). Fix the issue by setting nr_to_write to LONG_MAX in WB_SYNC_ALL mode. We do not need nr_to_write in WB_SYNC_ALL mode anyway since livelock avoidance is done differently for it. After this patch, program from http://lkml.org/lkml/2010/10/24/154 is no longer able to stall sync forever. Signed-off-by: Jan Kara <jack@xxxxxxx> --- fs/fs-writeback.c | 18 ++++++++++++++---- 1 files changed, 14 insertions(+), 4 deletions(-) Fengguang, I've been testing with those writeback fixes you reposted a few days ago and I've been able to still reproduce livelocks with Jan Engelhard's test case. Using writeback tracing I've tracked the problem to the above and with this patch, sync finishes OK (well, it still takes about 15 minutes but that's about expected time given the throughput I see to the disk - the test case randomly dirties pages in a huge file). So could you please add this patch to the previous two send them to Jens for inclusion? diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 6b4d02a..d5873a6 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -629,6 +629,7 @@ static long wb_writeback(struct bdi_writeback *wb, }; unsigned long oldest_jif; long wrote = 0; + long write_chunk; struct inode *inode; if (wbc.for_kupdate) { @@ -640,6 +641,15 @@ static long wb_writeback(struct bdi_writeback *wb, wbc.range_start = 0; wbc.range_end = LLONG_MAX; } + /* + * 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; wbc.wb_start = jiffies; /* livelock avoidance */ for (;;) { @@ -665,7 +675,7 @@ static long wb_writeback(struct bdi_writeback *wb, break; wbc.more_io = 0; - wbc.nr_to_write = MAX_WRITEBACK_PAGES; + wbc.nr_to_write = write_chunk; wbc.pages_skipped = 0; trace_wbc_writeback_start(&wbc, wb->bdi); @@ -675,8 +685,8 @@ static long wb_writeback(struct bdi_writeback *wb, writeback_inodes_wb(wb, &wbc); trace_wbc_writeback_written(&wbc, wb->bdi); - work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; - wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write; + work->nr_pages -= write_chunk - wbc.nr_to_write; + wrote += write_chunk - wbc.nr_to_write; /* * If we consumed everything, see if we have more @@ -691,7 +701,7 @@ static long wb_writeback(struct bdi_writeback *wb, /* * Did we write something? Try for more */ - if (wbc.nr_to_write < MAX_WRITEBACK_PAGES) + if (wbc.nr_to_write < write_chunk) continue; /* * Nothing written. Wait for some inode to -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
