• Nick Piggin's avatar
    Fix page writeback thinko, causing Berkeley DB slowdown · 3a4c6800
    Nick Piggin authored
    A bug was introduced into write_cache_pages cyclic writeout by commit
    31a12666 ("mm: write_cache_pages cyclic
    fix").  The intention (and comments) is that we should cycle back and
    look for more dirty pages at the beginning of the file if there is no
    more work to be done.
    
    But the !done condition was dropped from the test.  This means that any
    time the page writeout loop breaks (eg.  due to nr_to_write == 0), we
    will set index to 0, then goto again.  This will set done_index to
    index, then find done is set, so will proceed to the end of the
    function.  When updating mapping->writeback_index for cyclic writeout,
    we now use done_index == 0, so we're always cycling back to 0.
    
    This seemed to be causing random mmap writes (slapadd and iozone) to
    start writing more pages from the LRU and writeout would slowdown, and
    caused bugzilla entry
    
    	http://bugzilla.kernel.org/show_bug.cgi?id=12604
    
    about Berkeley DB slowing down dramatically.
    
    With this patch, iozone random write performance is increased nearly
    5x on my system (iozone -B -r 4k -s 64k -s 512m -s 1200m on ext2).
    Signed-off-by: default avatarNick Piggin <npiggin@suse.de>
    Reported-and-tested-by: default avatarJan Kara <jack@suse.cz>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    3a4c6800
page-writeback.c 39.7 KB