linux-kernel - [PATCH] PM / Hibernate: Fix free_unnecessary

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Date:	Wed, 6 Jul 2011 20:46:23 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Matthew Garrett <mjg@...hat.com>
Subject: [PATCH] PM / Hibernate: Fix free_unnecessary_pages()

From: Rafael J. Wysocki <rjw@...k.pl>

There is a bug in free_unnecessary_pages() that causes it to
attempt to free too many pages in some cases, which triggers the
BUG_ON() in memory_bm_clear_bit() for copy_bm.  Namely, if
count_data_pages() is initially greater than alloc_normal, we get
to_free_normal equal to 0 and "save" greater from 0.  In that case,
if the sum of "save" and count_highmem_pages() is greater than
alloc_highmem, we subtract a positive number from to_free_normal.
Hence, since to_free_normal was 0 before the subtraction and is
an unsigned int, the result is converted to a huge positive number
that is used as the number of pages to free.

Fix this bug by checking if to_free_normal is actually greater
than or equal to the number we're going to subtract from it.

Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
Reported-and-tested-by: Matthew Garrett <mjg@...hat.com>
Cc: stable@...nel.org
---
 kernel/power/snapshot.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Index: linux-2.6/kernel/power/snapshot.c
===================================================================
--- linux-2.6.orig/kernel/power/snapshot.c
+++ linux-2.6/kernel/power/snapshot.c
@@ -1211,7 +1211,11 @@ static void free_unnecessary_pages(void)
 		to_free_highmem = alloc_highmem - save;
 	} else {
 		to_free_highmem = 0;
-		to_free_normal -= save - alloc_highmem;
+		save -= alloc_highmem;
+		if (to_free_normal > save)
+			to_free_normal -= save;
+		else
+			to_free_normal = 0;
 	}

 	memory_bm_position_reset(&copy_bm);
--
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