[PATCH 4/8] fs: shrink only (SB_ACTIVE|SB_BORN) superblocks in super_cache_scan()

[Qi Zheng <qi.zheng@linux.dev>]

From: Kirill Tkhai <tkhai@ya.ru>

This patch prepares superblock shrinker for delayed unregistering.
It makes super_cache_scan() avoid shrinking of not active superblocks.
SB_ACTIVE is used as such the indicator. In case of superblock is not
active, super_cache_scan() just exits with SHRINK_STOP as result.

Note, that SB_ACTIVE is cleared in generic_shutdown_super() and this
is made under the write lock of s_umount. Function super_cache_scan()
also takes the read lock of s_umount, so it can't skip this flag cleared.

SB_BORN check is added to super_cache_scan() just for uniformity
with super_cache_count(), while super_cache_count() received SB_ACTIVE
check just for uniformity with super_cache_scan().

After this patch super_cache_scan() becomes to ignore unregistering
superblocks, so this function is OK with splitting unregister_shrinker().
Next patches prepare super_cache_count() to follow this way.

Signed-off-by: Kirill Tkhai <tkhai@ya.ru>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 fs/super.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index 2ce4c72720f3..2ce54561e82e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -79,6 +79,11 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
 	if (!trylock_super(sb))
 		return SHRINK_STOP;
 
+	if ((sb->s_flags & (SB_BORN|SB_ACTIVE)) != (SB_BORN|SB_ACTIVE)) {
+		freed = SHRINK_STOP;
+		goto unlock;
+	}
+
 	if (sb->s_op->nr_cached_objects)
 		fs_objects = sb->s_op->nr_cached_objects(sb, sc);
 
@@ -110,6 +115,7 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
 		freed += sb->s_op->free_cached_objects(sb, sc);
 	}
 
+unlock:
 	up_read(&sb->s_umount);
 	return freed;
 }
@@ -136,7 +142,7 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 	 * avoid this situation, so do the same here. The memory barrier is
 	 * matched with the one in mount_fs() as we don't hold locks here.
 	 */
-	if (!(sb->s_flags & SB_BORN))
+	if ((sb->s_flags & (SB_BORN|SB_ACTIVE)) != (SB_BORN|SB_ACTIVE))
 		return 0;
 	smp_rmb();
 
-- 
2.30.2