On Tue, 09 Nov 2010 07:09:20 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > From: Jan Kara <jack@xxxxxxx> > > 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. No, nr_to_write can only be negative if the filesystem wrote back more pages than requested. > 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). Well we shouldn't have asked the function to write LONG_MAX pages then! The way this used to work was to try to write back N=(total dirty pages + total unstable pages + various fudge factors) to each superblock. So each superblock will get fully written back unless someone is madly writing to it. If that _is_ happening then we'll write a large amount of data to it and will then give up and move onto the next superblock. But the "large amount of data" is constrained to a sane upper limit: total amount of dirty memory plus fudge factors. Increasing that sane upper limit to an insane 2^63-1 pages will *of course* cause sync() to livelock. Why was that sane->insane change made? > 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. Here the changelog should spell out what "done differently" means. Because I really am unsure what is begin referred to. I don't really see how this patch changes anything. For WB_SYNC_ALL requests the code will still try to write out 2^63 pages, only it does it all in a single writeback_inodes_wb() call. What prevents that call itself from getting livelocked? Perhaps the unmentioned problem here is that each call to writeback_inodes_wb(MAX_WRITEBACK_PAGES) will restart its walk across the inode lists. So instead of giving up on a being-written-to-file, we continuously revisit it again and again and again. Correct? If so, please add the description. If incorrect, please add the description as well ;) Root cause time: it's those damn per-sb inode lists *again*. They're just awful. We need some data structure there which is more amenable to being iterated over. Something against which we can store cursors, for a start. -- 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
