linux-kernel - [PATCH v2 2/3] sbitmap: fix strict round-robin non-wrap with hint

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Message-ID: <20230727152020.3633009-2-chengming.zhou@linux.dev>
Date:   Thu, 27 Jul 2023 23:20:19 +0800
From:   chengming.zhou@...ux.dev
To:     axboe@...nel.dk, osandov@...com, ming.lei@...hat.com,
        kbusch@...nel.org, krisman@...e.de
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhouchengming@...edance.com
Subject: [PATCH v2 2/3] sbitmap: fix strict round-robin non-wrap with hint > 0

From: Chengming Zhou <zhouchengming@...edance.com>

If we have alloc_hint > 0 and don't wrap, we need to recheck
sb->map[index] with hint == 0 to exhaust the map.

For example: we have 8 words to search, index == 4, hint > 0

In sb->map[4] word, we only search [hint, 63] bits in the first
word search. Then we search left 7 words, the sb->map[4] [0, hint-1]
bits aren't searched.

This patch fix it by searching 9 words in this case, so we will
search sb->map[4] the second time at last with hint == 0.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 lib/sbitmap.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index ac4027884765..6e098a46be26 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -199,10 +199,18 @@ static int sbitmap_find_bit(struct sbitmap *sb,
 			    unsigned int alloc_hint,
 			    bool wrap)
 {
+	unsigned int map_nr = sb->map_nr;
 	unsigned int i;
 	int nr = -1;
 
-	for (i = 0; i < sb->map_nr; i++) {
+	/*
+	 * If we have alloc_hint > 0 and don't wrap, we need to
+	 * recheck sb->map[index] with hint == 0 to exhaust the map.
+	 */
+	if (alloc_hint && !wrap)
+		map_nr += 1;
+
+	for (i = 0; i < map_nr; i++) {
 		nr = sbitmap_find_bit_in_word(&sb->map[index],
 					      min_t(unsigned int,
 						    __map_depth(sb, index),
-- 
2.41.0