gfs2: less aggressive low-memory log flushing

It turns out that for some workloads, the fix in commit b74cd55
("gfs2: low-memory forced flush fixes") causes the number of forced log
flushes to increase to a degree that the overall filesystem performance
drops significantly.  Address that by forcing a log flush only when
gfs2_writepages cannot make any progress rather than when it cannot make
"enough" progress.

Fixes: b74cd55 ("gfs2: low-memory forced flush fixes")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: Andreas Gruenbacher

fs/gfs2/aops.c

@@ -158,6 +158,7 @@ static int gfs2_writepages(struct address_space *mapping,
 			   struct writeback_control *wbc)
 {
 	struct gfs2_sbd *sdp = gfs2_mapping2sbd(mapping);
+	long initial_nr_to_write = wbc->nr_to_write;
 	struct iomap_writepage_ctx wpc = {
 		.inode		= mapping->host,
 		.wbc		= wbc,
@@ -166,13 +167,13 @@ static int gfs2_writepages(struct address_space *mapping,
 	int ret;
 
 	/*
-	 * Even if we didn't write enough pages here, we might still be holding
+	 * Even if we didn't write any pages here, we might still be holding
 	 * dirty pages in the ail. We forcibly flush the ail because we don't
 	 * want balance_dirty_pages() to loop indefinitely trying to write out
 	 * pages held in the ail that it can't find.
 	 */
 	ret = iomap_writepages(&wpc);
-	if (ret == 0 && wbc->nr_to_write > 0)
+	if (ret == 0 && wbc->nr_to_write == initial_nr_to_write)
 		set_bit(SDF_FORCE_AIL_FLUSH, &sdp->sd_flags);
 	return ret;
 }