On Sun, 2009-10-11 at 10:28 +0800, Wu Fengguang wrote: > > Note that the total limit check itself may not be sufficient. For > example, there are no nr_writeback limit for NFS (and maybe btrfs) > after removing the congestion waits. Therefore it is very possible > > nr_writeback => dirty_thresh > nr_dirty => 0 > > which is obviously undesirable: everything newly dirtied are soon put > to writeback. It violates the 30s expire time and the background > threshold rules, and will hurt write-and-truncate operations (ie. temp > files). > > So the better solution would be to impose a nr_writeback limit for > every filesystem that didn't already have one (the block io queue). > NFS used to have that limit with congestion_wait, but now we need > to do a wait queue for it. > > With the nr_writeback wait queue, it can be guaranteed that once > balance_dirty_pages() asks for writing 1500 pages, it will be done > with necessary sleeping in the bdi flush thread. So we can safely > remove the loop and double checking of global dirty limit in > balance_dirty_pages(). nr_reclaim = nr_dirty + nr_writeback + nr_unstable, so anything calling into balance_dirty_pages() would still block on seeing such large amounts of nr_writeback. Having the constraint nr_dirty + nr_writeback + nr_unstable < dirty_thresh should ensure we never have nr_writeback > dirty_thresh, simply because you cannot dirty more, which then cannot be converted to more writeback. Or am I missing something? -- 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
