On Mon, Aug 31 2009, Jens Axboe wrote: > > I traced the calls to ext4_da_writepages() using ftrace, and found this: > > > > flush-8:16-1829 [001] 23.416351: ext4_da_writepages: dev sdb ino 12 nr_t_write 32759 pages_skipped 0 range_start 0 range_end 0 nonblocking 0 for_kupdate 0 for_reclaim 0 for_writepages 1 range_cyclic 1 > > flush-8:16-1829 [000] 25.939354: ext4_da_writepages: dev sdb ino 12 nr_t_write 32768 pages_skipped 0 range_start 0 range_end 0 nonblocking 0 for_kupdate 0 for_reclaim 0 for_writepages 1 range_cyclic 1 > > flush-8:16-1829 [000] 25.939486: ext4_da_writepages: dev sdb ino 13 nr_t_write 32759 pages_skipped 0 range_start 134180864 range_end 9223372036854775807 nonblocking 0 for_kupdate 0 for_reclaim 0 for_writepages 1 range_cyclic 1 > > flush-8:16-1829 [000] 27.055687: ext4_da_writepages: dev sdb ino 12 nr_t_write 32768 pages_skipped 0 range_start 0 range_end 0 nonblocking 0 for_kupdate 0 for_reclaim 0 for_writepages 1 range_cyclic 1 > > flush-8:16-1829 [000] 27.055691: ext4_da_writepages: dev sdb ino 13 nr_t_write 32768 pages_skipped 0 range_start 0 range_end 0 nonblocking 0 for_kupdate 0 for_reclaim 0 for_writepages 1 range_cyclic 1 > > flush-8:16-1829 [000] 27.878708: ext4_da_writepages: dev sdb ino 13 nr_t_write 32768 pages_skipped 0 range_start 0 range_end 0 nonblocking 0 for_kupdate 0 for_reclaim 0 for_writepages 1 range_cyclic 1 > > > > The *first* time the per-bdi code called writepages on the second file > > (test2, inode #13), range_start was 134180864 (which, curiously > > enough, is 4096*32759, which was the value of nr_to_write passed to > > ext4_da_writepages). Given that the inode only had 32768 pages, the > > fact that apparently *some* codepath called ext4_da_writepages > > starting at logical block 32759, with nr_to_write set to 32759, seems > > very curious indeed. That doesn't seem right at all. It's late, so I > > won't try to trace it down now; plus which it's your code so I figure > > you can probably figure it out faster.... > > Interesting, needs checking up on. I've prepared a v14 patchset today, > perhaps (if you have time), you can see if it reproduces there? I'm > running some performance tests today, but will make a note to look into > this after that. It's because ext4 writepages sets ->range_start and wb_writeback() is range cyclic, then the next iteration will have the previous end point as the starting point. Looks like we need to clear ->range_start in wb_writeback(), the better place is probably to do that in fs/fs-writeback.c:generic_sync_wb_inodes() right after the writeback_single_inode() call. This, btw, should be no different than the current code, weird/correct or not :-) -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
