linux-ext4 - [PATCH] fs/ext4: fix potential endless loop in ext4_mb_discard_group

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Date:	Mon, 31 Aug 2015 17:46:24 +0800
From:	Zhang Zhen <zhenzhang.zhang@...wei.com>
To:	"tytso@....edu" <tytso@....edu>, Jan Kara <jack@...e.cz>,
	<adilger.kernel@...ger.ca>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: [PATCH] fs/ext4: fix potential endless loop in ext4_mb_discard_group_preallocations()

In ext4_mb_discard_group_preallocations(), if free is always less than needed,
and some PAs are always used in every loop, it will be endless loop.

Here we pick a random value to limit the max number of loop.

Signed-off-by: Zhang Zhen <zhenzhang.zhang@...wei.com>
---
 fs/ext4/mballoc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 34b610e..553fbde 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3836,6 +3836,7 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 	int err;
 	int busy = 0;
 	int free = 0;
+	int tried = 0;

 	mb_debug(1, "discard preallocation for group %u\n", group);

@@ -3886,9 +3887,11 @@ repeat:
 		list_add(&pa->u.pa_tmp_list, &list);
 	}

-	/* if we still need more blocks and some PAs were used, try again */
-	if (free < needed && busy) {
+	/* if we still need more blocks and some PAs were used, try again,
+	   here 20 is a ramdon value. */
+	if (free < needed && busy && tried < 20) {
 		busy = 0;
+		tried++;
 		ext4_unlock_group(sb, group);
 		cond_resched();
 		goto repeat;
-- 
1.9.1


.




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