linux-kernel - [PATCH RFC 06/10] fs: Shrink only (SB_ACTIVE|SB_BORN) superblocks in super_cache

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Date:   Tue, 07 Aug 2018 18:38:36 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
        rafael@...nel.org, viro@...iv.linux.org.uk,
        darrick.wong@...cle.com, paulmck@...ux.vnet.ibm.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        hughd@...gle.com, shuah@...nel.org, robh@...nel.org,
        ulf.hansson@...aro.org, aspriel@...il.com,
        vivek.gautam@...eaurora.org, robin.murphy@....com, joe@...ches.com,
        heikki.krogerus@...ux.intel.com, ktkhai@...tuozzo.com,
        sfr@...b.auug.org.au, vdavydov.dev@...il.com, mhocko@...e.com,
        chris@...is-wilson.co.uk, penguin-kernel@...ove.SAKURA.ne.jp,
        aryabinin@...tuozzo.com, willy@...radead.org, ying.huang@...el.com,
        shakeelb@...gle.com, jbacik@...com, mingo@...nel.org,
        mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH RFC 06/10] fs: Shrink only (SB_ACTIVE|SB_BORN) superblocks
 in super_cache_scan()

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 s_umount mutex. Function super_cache_scan() also takes
the mutex, 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 <ktkhai@...tuozzo.com>
---
 fs/super.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index 457834278e37..9222cfc196bf 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();