[PATCH v4 1/2] md/raid5: track disks array size to fix extra_page leak on error paths

[ghuicao@163.com]

From: Cao Guanghui <caoguanghui@kylinos.cn>

free_conf() iterates conf->pool_size entries to free extra_page
allocations, but pool_size may not reflect the actual size of the
conf->disks array.  Two scenarios cause a mismatch:

1. setup_conf() early abort: pool_size is 0 (not yet set by
   grow_stripes) but conf->disks has max_disks entries with
   extra_page allocated.  The loop iterates 0 times, leaking all
   pages.

2. resize_stripes() Step 4 failure: conf->disks was replaced with
   a newsize-entry array in Step 3, but pool_size is only updated
   on success.  The loop iterates pool_size (old, smaller value)
   times, leaking (newsize - pool_size) pages.

Add a dedicated disks_cnt field to track the actual number of
entries in conf->disks.  Set it immediately after each allocation
or replacement (in setup_conf and resize_stripes Step 3, where the
array is safely stalled with no concurrent access), and use it in
free_conf() instead of pool_size.

This leaves pool_size untouched, preserving the check_reshape()
retry behavior that depends on pool_size only being updated on
full success.

Fixes: d7bd398e97f2 ("md/r5cache: handle alloc_page failure")
Cc: stable@vger.kernel.org
Signed-off-by: Cao Guanghui <caoguanghui@kylinos.cn>
---
 drivers/md/raid5.c | 4 +++-
 drivers/md/raid5.h | 1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2642,6 +2642,7 @@ static int resize_stripes(struct r5conf *conf, int newsize)
 		} else {
 			kfree(conf->disks);
 			conf->disks = ndisks;
+			conf->disks_cnt = newsize;
 		}
 	} else
 		err = -ENOMEM;
@@ -7552,7 +7553,7 @@ static void free_conf(struct r5conf *conf)
 	free_thread_groups(conf);
 	shrink_stripes(conf);
 	raid5_free_percpu(conf);
-	for (i = 0; i < conf->pool_size; i++)
+	for (i = 0; i < conf->disks_cnt; i++)
 		if (conf->disks[i].extra_page)
 			put_page(conf->disks[i].extra_page);
 	kfree(conf->disks);
@@ -7733,7 +7734,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 
 	if (!conf->disks)
 		goto abort;
-
+	conf->disks_cnt = max_disks;
 	for (i = 0; i < max_disks; i++) {
 		conf->disks[i].extra_page = alloc_page(GFP_KERNEL);
 		if (!conf->disks[i].extra_page)
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -667,6 +667,7 @@ struct r5conf {
 	unsigned long		cache_state;
 	struct shrinker		*shrinker;
 	int			pool_size; /* number of disks in stripeheads in pool */
+	int			disks_cnt; /* number of entries in disks[] array */
 	spinlock_t		device_lock;
 	struct disk_info	*disks;
 	struct bio_set		bio_split;
-- 
2.34.1