• Jun'ichi Nomura's avatar
    writeback: make mapping->writeback_index to point to the last written page · cf15b07c
    Jun'ichi Nomura authored
    For range-cyclic writeback (e.g.  kupdate), the writeback code sets a
    continuation point of the next writeback to mapping->writeback_index which
    is set the page after the last written page.  This happens so that we
    evenly write the whole file even if pages in it get continuously
    redirtied.
    
    However, in some cases, sequential writer is writing in the middle of the
    page and it just redirties the last written page by continuing from that.
    For example with an application which uses a file as a big ring buffer we
    see:
    
    [1st writeback session]
           ...
           flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898514 + 8
           flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898522 + 8
           flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898530 + 8
           flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898538 + 8
           flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898546 + 8
         kworker/0:1-11    4571: block_rq_issue: 8,0 W 0 () 94898514 + 40
    >>     flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898554 + 8
    >>     flush-8:0-2743  4571: block_rq_issue: 8,0 W 0 () 94898554 + 8
    
    [2nd writeback session after 35sec]
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898562 + 8
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898570 + 8
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898578 + 8
           ...
         kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94898562 + 640
         kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94899202 + 72
           ...
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899962 + 8
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899970 + 8
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899978 + 8
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899986 + 8
           flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899994 + 8
         kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94899962 + 40
    >>     flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898554 + 8
    >>     flush-8:0-2743  4606: block_rq_issue: 8,0 W 0 () 94898554 + 8
    
    So we seeked back to 94898554 after we wrote all the pages at the end of
    the file.
    
    This extra seek seems unnecessary.  If we continue writeback from the last
    written page, we can avoid it and do not cause harm to other cases.  The
    original intent of even writeout over the whole file is preserved and if
    the page does not get redirtied pagevec_lookup_tag() just skips it.
    
    As an exceptional case, when I/O error happens, set done_index to the next
    page as the comment in the code suggests.
    Tested-by: default avatarWu Fengguang <fengguang.wu@intel.com>
    Signed-off-by: default avatarJun'ichi Nomura <j-nomura@ce.jp.nec.com>
    Signed-off-by: default avatarJan Kara <jack@suse.cz>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    cf15b07c
page-writeback.c 39.6 KB