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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "Mike Snitzer" <snitzer@redhat.com>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
Message-ID: <lsq.1544392233.545839705@decadent.org.uk>
Subject: [PATCH 3.16 115/328] dm cache metadata: save in-core
 policy_hint_size to on-disk superblock
In-Reply-To: <lsq.1544392232.713909046@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mike Snitzer <snitzer@redhat.com>

commit fd2fa95416188a767a63979296fa3e169a9ef5ec upstream.

policy_hint_size starts as 0 during __write_initial_superblock().  It
isn't until the policy is loaded that policy_hint_size is set in-core
(cmd->policy_hint_size).  But it never got recorded in the on-disk
superblock because __commit_transaction() didn't deal with transfering
the in-core cmd->policy_hint_size to the on-disk superblock.

The in-core cmd->policy_hint_size gets initialized by metadata_open()'s
__begin_transaction_flags() which re-reads all superblock fields.
Because the superblock's policy_hint_size was never properly stored, when
the cache was created, hints_array_available() would always return false
when re-activating a previously created cache.  This means
__load_mappings() always considered the hints invalid and never made use
of the hints (these hints served to optimize).

Another detremental side-effect of this oversight is the cache_check
utility would fail with: "invalid hint width: 0"

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 drivers/md/dm-cache-metadata.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/md/dm-cache-metadata.c
+++ b/drivers/md/dm-cache-metadata.c
@@ -324,7 +324,7 @@ static int __write_initial_superblock(st
 	disk_super->version = cpu_to_le32(MAX_CACHE_VERSION);
 	memset(disk_super->policy_name, 0, sizeof(disk_super->policy_name));
 	memset(disk_super->policy_version, 0, sizeof(disk_super->policy_version));
-	disk_super->policy_hint_size = 0;
+	disk_super->policy_hint_size = cpu_to_le32(0);
 
 	__copy_sm_root(cmd, disk_super);
 
@@ -635,6 +635,7 @@ static int __commit_transaction(struct d
 	disk_super->policy_version[0] = cpu_to_le32(cmd->policy_version[0]);
 	disk_super->policy_version[1] = cpu_to_le32(cmd->policy_version[1]);
 	disk_super->policy_version[2] = cpu_to_le32(cmd->policy_version[2]);
+	disk_super->policy_hint_size = cpu_to_le32(cmd->policy_hint_size);
 
 	disk_super->read_hits = cpu_to_le32(cmd->stats.read_hits);
 	disk_super->read_misses = cpu_to_le32(cmd->stats.read_misses);