[PATCH v2] btrfs: add framework to handle device flush error as a volume

[Anand Jain <anand.jain@oracle.com>]

This adds comments to the flush error handling part of
the code, and hopes to maintain the same logic with a
framework which can be used to handle the errors at the
volume level.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v2:
 fix -ENOMEM at two places
 add code readability changes in check_barrier_error()

 fs/btrfs/disk-io.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++----
 fs/btrfs/volumes.h |  1 +
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index e064913fa62f..2f0d0688e0a6 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3625,6 +3625,10 @@ static int write_dev_flush(struct btrfs_device *device, int wait)
 	if (wait) {
 		bio = device->flush_bio;
 		if (!bio)
+			/*
+			 * This means the alloc has failed with ENOMEM, however
+			 * here we return 0, as its not a device error.
+			 */
 			return 0;
 
 		wait_for_completion(&device->flush_wait);
@@ -3664,6 +3668,32 @@ static int write_dev_flush(struct btrfs_device *device, int wait)
 	return 0;
 }
 
+static int check_barrier_error(struct btrfs_fs_devices *fsdevs)
+{
+	int submit_flush_error = 0;
+	int dev_flush_error = 0;
+	struct btrfs_device *dev;
+	int tolerance;
+
+	list_for_each_entry_rcu(dev, &fsdevs->devices, dev_list) {
+		if (!dev->bdev) {
+			submit_flush_error++;
+			dev_flush_error++;
+			continue;
+		}
+		if (dev->last_flush_error == -ENOMEM)
+			submit_flush_error++;
+		if (dev->last_flush_error && dev->last_flush_error != -ENOMEM)
+			dev_flush_error++;
+	}
+
+	tolerance = fsdevs->fs_info->num_tolerated_disk_barrier_failures;
+	if (submit_flush_error > tolerance || dev_flush_error > tolerance)
+		return -EIO;
+
+	return 0;
+}
+
 /*
  * send an empty flush down to each device in parallel,
  * then wait for them
@@ -3691,6 +3721,7 @@ static int barrier_all_devices(struct btrfs_fs_info *info)
 		ret = write_dev_flush(dev, 0);
 		if (ret)
 			errors_send++;
+		dev->last_flush_error = ret;
 	}
 
 	/* wait for all the barriers */
@@ -3705,12 +3736,30 @@ static int barrier_all_devices(struct btrfs_fs_info *info)
 			continue;
 
 		ret = write_dev_flush(dev, 1);
-		if (ret)
+		if (ret) {
+			dev->last_flush_error = ret;
 			errors_wait++;
+		}
+	}
+
+	/*
+	 * Try hard in case of flush. Lets say, in RAID1 we have
+	 * the following situation
+	 *  dev1: EIO dev2: ENOMEM
+	 * this is not a fatal error as we hope to recover from
+	 * ENOMEM in the next attempt to flush.
+	 * But the following is considered as fatal
+	 *  dev1: ENOMEM dev2: ENOMEM
+	 *  dev1: bdev == NULL dev2: ENOMEM
+	 */
+	if (errors_send || errors_wait) {
+		/*
+		 * At some point we need the status of all disks
+		 * to arrive at the volume status. So error checking
+		 * is being pushed to a separate loop.
+		 */
+		return check_barrier_error(info->fs_devices);
 	}
-	if (errors_send > info->num_tolerated_disk_barrier_failures ||
-	    errors_wait > info->num_tolerated_disk_barrier_failures)
-		return -EIO;
 	return 0;
 }
 
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index cfb817384cb5..9bb248b3fa0e 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -87,6 +87,7 @@ struct btrfs_device {
 	int offline;
 	int can_discard;
 	int is_tgtdev_for_dev_replace;
+	int last_flush_error;
 
 #ifdef __BTRFS_NEED_DEVICE_DATA_ORDERED
 	seqcount_t data_seqcount;
-- 
2.10.0