[PATCH] writeback: skip balance_dirty_pages() for in-memory fs

[Wu Fengguang]

This avoids unnecessary checks and dirty throttling on tmpfs/ramfs.

It also prevents

[  388.126563] BUG: unable to handle kernel NULL pointer dereference at 0000000000000050

in the balance_dirty_pages tracepoint, which will call

	dev_name(mapping->backing_dev_info->dev)

but shmem_backing_dev_info.dev is NULL.

CC: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/page-writeback.c |    3 +++
 1 file changed, 3 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2010-12-17 19:09:19.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-12-17 19:09:22.000000000 +0800
@@ -899,6 +899,9 @@ void balance_dirty_pages_ratelimited_nr(
 {
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 
+	if (!bdi_cap_account_dirty(bdi))
+		return;
+
 	current->nr_dirtied += nr_pages_dirtied;
 
 	if (unlikely(!current->nr_dirtied_pause))
--
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