writeback: pay attention to wbc->nr_to_write in write_cache_pages

If a filesystem writes more than one page in ->writepage, write_cache_pages
fails to notice this and continues to attempt writeback when wbc->nr_to_write
has gone negative - this trace was captured from XFS:

    wbc_writeback_start: towrt=1024
    wbc_writepage: towrt=1024
    wbc_writepage: towrt=0
    wbc_writepage: towrt=-1
    wbc_writepage: towrt=-5
    wbc_writepage: towrt=-21
    wbc_writepage: towrt=-85

This has adverse effects on filesystem writeback behaviour. write_cache_pages()
needs to terminate after a certain number of pages are written, not after a
certain number of calls to ->writepage are made.  This is a regression
introduced by 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4 ("vfs: Add
no_nrwrite_index_update writeback control flag"), but cannot be reverted
directly due to subsequent bug fixes that have gone in on top of it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 9 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index f64134653a8c..d63ef8f9609f 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -56,15 +56,6 @@ struct writeback_control {
         unsigned for_reclaim:1;         /* Invoked from the page allocator */
         unsigned range_cyclic:1;        /* range_start is cyclic */
         unsigned more_io:1;             /* more io to be dispatched */
-        /*
-         * write_cache_pages() won't update wbc->nr_to_write and
-         * mapping->writeback_index if no_nrwrite_index_update
-         * is set.  write_cache_pages() may write more than we
-         * requested and we want to make sure nr_to_write and
-         * writeback_index are updated in a consistent manner
-         * so we use a single control to update them
-         */
-        unsigned no_nrwrite_index_update:1;
 };
 
 /*