mm: fix writeback_in_progress()

Commit 83ba7b071f3 ("writeback: simplify the write back thread queue")
broke writeback_in_progress() as in that commit we started to remove work
items from the list at the moment we start working on them and not at the
moment they are finished.  Thus if the flusher thread was doing some work
but there was no other work queued, writeback_in_progress() returned
false.  This could in particular cause unnecessary queueing of background
writeback from balance_dirty_pages() or writeout work from
writeback_sb_if_idle().

This patch fixes the problem by introducing a bit in the bdi state which
indicates that the flusher thread is processing some work and uses this
bit for writeback_in_progress() test.

NOTE: Both callsites of writeback_in_progress() (namely,
writeback_inodes_sb_if_idle() and balance_dirty_pages()) would actually
need a different information than what writeback_in_progress() provides.
They would need to know whether *the kind of writeback they are going to
submit* is already queued.  But this information isn't that simple to
provide so let's fix writeback_in_progress() for the time being.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Acked-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c

@@ -68,7 +68,7 @@ int nr_pdflush_threads;
  */
 int writeback_in_progress(struct backing_dev_info *bdi)
 {
-	return !list_empty(&bdi->work_list);
+	return test_bit(BDI_writeback_running, &bdi->state);
 }
 
 static void bdi_queue_work(struct backing_dev_info *bdi,
@@ -740,6 +740,7 @@ long wb_do_writeback(struct bdi_writeback *wb, int force_wait)
 	struct wb_writeback_work *work;
 	long wrote = 0;
 
+	set_bit(BDI_writeback_running, &wb->bdi->state);
 	while ((work = get_next_work_item(bdi)) != NULL) {
 		/*
 		 * Override sync mode, in case we must wait for completion
@@ -766,6 +767,7 @@ long wb_do_writeback(struct bdi_writeback *wb, int force_wait)
 	 * Check for periodic writeback, kupdated() style
 	 */
 	wrote += wb_check_old_data_flush(wb);
+	clear_bit(BDI_writeback_running, &wb->bdi->state);
 
 	return wrote;
 }

include/linux/backing-dev.h

@@ -31,6 +31,7 @@ enum bdi_state {
 	BDI_async_congested,	/* The async (write) queue is getting full */
 	BDI_sync_congested,	/* The sync queue is getting full */
 	BDI_registered,		/* bdi_register() was done */
+	BDI_writeback_running,	/* Writeback is in progress */
 	BDI_unused,		/* Available bits start here */
 };