From: David Rientjes <rientjes@google.com>
Date: Wed, 6 Aug 2014 23:07:31 +0000 (-0700)
Subject: mm, writeback: prevent race when calculating dirty limits
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=9ef0a0ffa28edbf5c7cfa6be73b4ecb9896a3875;p=GitHub%2FLineageOS%2FG12%2Fandroid_kernel_amlogic_linux-4.9.git

mm, writeback: prevent race when calculating dirty limits

Setting vm_dirty_bytes and dirty_background_bytes is not protected by
any serialization.

Therefore, it's possible for either variable to change value after the
test in global_dirty_limits() to determine whether available_memory
needs to be initialized or not.

Always ensure that available_memory is properly initialized.

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e0c943014eb7..91d73ef1744d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -261,14 +261,11 @@ static unsigned long global_dirtyable_memory(void)
  */
 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
+	const unsigned long available_memory = global_dirtyable_memory();
 	unsigned long background;
 	unsigned long dirty;
-	unsigned long uninitialized_var(available_memory);
 	struct task_struct *tsk;
 
-	if (!vm_dirty_bytes || !dirty_background_bytes)
-		available_memory = global_dirtyable_memory();
-
 	if (vm_dirty_bytes)
 		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
 	else