[PATCH v2 0/3] Misc volume patch set part2

[PATCH v2 0/3] Misc volume patch set part2

[Anand Jain]

The patch set (2/3 and 3/3) adds helper function to deduce the num_device
(which is the number of the devices when device is mounted, this excludes
the seed device however includes the replacing target). We need to know
the num_device that actually belongs to the FSID without the replacing
target (if it exists) when we are balancing and or when we are trying to
device delete. In both of these cases, it would be wrong to consider
replacing target when reallocating the blockgroups.

This patch as such does not change any of the logic which is already in
the original code, but instead it would bring such a logic at two
locations into a single place/function. But to do that as these two
location uses either WARN_ON or BUG_ON to perform the safety catch so
the first patch does the cleanup and uses ASSERT at both of these
places. And now since these two sections of codes are identical, the 2nd
patch replaces them into a helper function.

Also though the check for num_device > 0 should actually be num_device >
1 OR instead, the check of num_device > 0 should have been after the
num_device--, since we have had already too many comments and added
confusions, I shall leave that part to be fixed at sometime later. So
that the cleanup here is a really a cleanup instead of chaning the
logic in a rough scale.

Last but not least, this patch has gone through a lot of comments on
either to use BUG or return -EINVAL for accidental fall of num_device
below 1, which I wasn't expecting so much of the concerns about
the use of BUG_ON/-EINVAL as its trivial because fundamentally a FS
can't be in the mounted state when num_device < 1.


Anand Jain (3):
  btrfs: drop uuid_mutex in btrfs_free_extra_devids()
  btrfs: assert for num_devices below 0
  btrfs: add helper btrfs_num_devices() to deduce num_devices

 fs/btrfs/volumes.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

-- 
2.7.0

[PATCH v4 1/3] btrfs: drop uuid_mutex in btrfs_free_extra_devids()

[Anand Jain]

btrfs_free_extra_devids() is called only in the mount context which
traverses through the fs_devices::devices and frees the orphan devices
devices in the given %fs_devices if any. As the search for the orphan
device is limited to fs_devices::devices so we don't need the global
uuid_mutex.

There can't be any mount-point based ioctl threads in this context as
the mount thread is not yet returned. But there can be the btrfs-control
based scan ioctls thread which calls device_list_add().

Here in the mount thread the fs_devices::opened is incremented way before
btrfs_free_extra_devids() is called and in the scan context the fs_devices
which are already opened neither be freed or alloc-able at
device_list_add().

But lets say you change the device-path and call the scan again, then scan
would update the new device path and this operation could race against the
btrfs_free_extra_devids() thread, which might be in the process of
free-ing the same device. So synchronize it by using the
device_list_mutex.

This scenario is a very corner case, and practically the scan and mount
are anyway serialized by the usage so unless the race is instrumented its
very difficult to achieve.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
---
v3->v4: As we traverse through the seed device, fs_device gets updated with
	the child seed fs_devices, so make sure we use the same fs_devices
	pointer for the mutex_unlock as used for the mutex_lock.
v2->v3: Update change log.
	(Currently device_list_add() is very lean on its device_list_mutex usage,
	a cleanup and fix is wip. Given the practicality of the above race
	condition this patch is good to merge).
v1->v2: replace uuid_mutex with device_list_mutex instead of delete.
	change log updated.

 fs/btrfs/volumes.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f4405e430da6..f277df935e7c 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -934,8 +934,9 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step)
 {
 	struct btrfs_device *device, *next;
 	struct btrfs_device *latest_dev = NULL;
+	struct btrfs_fs_devices *parent_fs_devices = fs_devices;
 
-	mutex_lock(&uuid_mutex);
+	mutex_lock(&parent_fs_devices->device_list_mutex);
 again:
 	/* This is the initialized path, it is safe to release the devices. */
 	list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) {
@@ -989,8 +990,7 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step)
 	}
 
 	fs_devices->latest_bdev = latest_dev->bdev;
-
-	mutex_unlock(&uuid_mutex);
+	mutex_unlock(&parent_fs_devices->device_list_mutex);
 }
 
 static void free_device_rcu(struct rcu_head *head)
-- 
2.7.0

[PATCH 1/2] btrfs: assert for num_devices below 0

[Anand Jain]

In preparation to add helper function to deduce the num_devices with
replace running, use assert instead of bug_on and warn_on.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/volumes.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index b6d9b6a6fba7..0062615a79be 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1877,7 +1877,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 	num_devices = fs_devices->num_devices;
 	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
 	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
-		WARN_ON(num_devices < 1);
+		ASSERT(num_devices > 0);
 		num_devices--;
 	}
 	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
@@ -3752,7 +3752,7 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 	num_devices = fs_info->fs_devices->num_devices;
 	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
 	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
-		BUG_ON(num_devices < 1);
+		ASSERT(num_devices > 0);
 		num_devices--;
 	}
 	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
-- 
2.7.0

[PATCH v5 2/2] btrfs: add helper btrfs_num_devices() to deduce num_devices

[Anand Jain]

When the replace is running the fs_devices::num_devices also includes
the replace device, however in some operations like device delete and
balance it needs the actual num_devices without the repalce devices, so
now the function btrfs_num_devices() just provides that.

And here is a scenario how balance and repalce items could co-exist.
Consider balance is started and paused, now start the replace
followed by a unmount or power-recycle of the system. During following
mount, the open_ctree() first restarts the balance so it must check for
the replace device otherwise our num_devices calculation will be wrong.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v4->v5: uses assert.
v3->v4: add comment and drop the inline (sorry missed it before)
v2->v3: update changelog with not so obvious balance and repalce
co-existance secnario
v1->v2: add comments

 fs/btrfs/volumes.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 0062615a79be..630f9ec158d0 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1863,6 +1863,21 @@ void btrfs_assign_next_active_device(struct btrfs_device *device,
 		fs_info->fs_devices->latest_bdev = next_device->bdev;
 }
 
+/* Returns btrfs_fs_devices::num_devices excluding replace device if any */
+static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
+{
+	u64 num_devices = fs_info->fs_devices->num_devices;
+
+	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
+	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
+		ASSERT(num_devices > 0);
+		num_devices--;
+	}
+	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
+
+	return num_devices;
+}
+
 int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 		u64 devid)
 {
@@ -1874,13 +1889,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 
 	mutex_lock(&uuid_mutex);
 
-	num_devices = fs_devices->num_devices;
-	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
-	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
-		ASSERT(num_devices > 0);
-		num_devices--;
-	}
-	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
+	num_devices = btrfs_num_devices(fs_info);
 
 	ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);
 	if (ret)
@@ -3749,13 +3758,8 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 		}
 	}
 
-	num_devices = fs_info->fs_devices->num_devices;
-	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
-	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
-		ASSERT(num_devices > 0);
-		num_devices--;
-	}
-	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
+	num_devices = btrfs_num_devices(fs_info);
+
 	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE | BTRFS_BLOCK_GROUP_DUP;
 	if (num_devices > 1)
 		allowed |= (BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1);
-- 
2.7.0

Re: [PATCH v5 2/2] btrfs: add helper btrfs_num_devices() to deduce num_devices

[David Sterba]

On Fri, Aug 10, 2018 at 01:53:21PM +0800, Anand Jain wrote:
> When the replace is running the fs_devices::num_devices also includes
> the replace device, however in some operations like device delete and
> balance it needs the actual num_devices without the repalce devices, so
> now the function btrfs_num_devices() just provides that.
> 
> And here is a scenario how balance and repalce items could co-exist.
> Consider balance is started and paused, now start the replace
> followed by a unmount or power-recycle of the system. During following
> mount, the open_ctree() first restarts the balance so it must check for
> the replace device otherwise our num_devices calculation will be wrong.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
> v4->v5: uses assert.
> v3->v4: add comment and drop the inline (sorry missed it before)
> v2->v3: update changelog with not so obvious balance and repalce
> co-existance secnario
> v1->v2: add comments
> 
>  fs/btrfs/volumes.c | 32 ++++++++++++++++++--------------
>  1 file changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 0062615a79be..630f9ec158d0 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1863,6 +1863,21 @@ void btrfs_assign_next_active_device(struct btrfs_device *device,
>  		fs_info->fs_devices->latest_bdev = next_device->bdev;
>  }
>  
> +/* Returns btrfs_fs_devices::num_devices excluding replace device if any */
> +static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
> +{
> +	u64 num_devices = fs_info->fs_devices->num_devices;
> +
> +	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
> +	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
> +		ASSERT(num_devices > 0);
> +		num_devices--;
> +	}
> +	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);

I'll move the assert with the updated condition here so it covers also
the non dev-replace case. Otherwise ok.

> +
> +	return num_devices;
> +}

Re: [PATCH 1/2] btrfs: assert for num_devices below 0

[David Sterba]

On Fri, Aug 10, 2018 at 01:53:20PM +0800, Anand Jain wrote:
> In preparation to add helper function to deduce the num_devices with
> replace running, use assert instead of bug_on and warn_on.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>

Ok for the updated condition as it's going to be used in the new helper.

Reviewed-by: David Sterba <dsterba@suse.com>

Re: [PATCH 1/2] btrfs: assert for num_devices below 0

[David Sterba]

On Fri, Aug 10, 2018 at 01:53:20PM +0800, Anand Jain wrote:
> In preparation to add helper function to deduce the num_devices with
> replace running, use assert instead of bug_on and warn_on.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>  fs/btrfs/volumes.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index b6d9b6a6fba7..0062615a79be 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1877,7 +1877,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
>  	num_devices = fs_devices->num_devices;
>  	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
>  	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
> -		WARN_ON(num_devices < 1);
> +		ASSERT(num_devices > 0);
>  		num_devices--;

I was about to merge the patch but this gave me another opportunity to
look at the code: the assertion should check for > 1. The value 1 of
num_devices is sligthly wrong here and would lead to 0 returned from the
function.

Re: [PATCH v2 0/3] Misc volume patch set part2

[David Sterba]

On Fri, Aug 10, 2018 at 01:53:18PM +0800, Anand Jain wrote:
> The patch set (2/3 and 3/3) adds helper function to deduce the num_device
> (which is the number of the devices when device is mounted, this excludes
> the seed device however includes the replacing target). We need to know
> the num_device that actually belongs to the FSID without the replacing
> target (if it exists) when we are balancing and or when we are trying to
> device delete. In both of these cases, it would be wrong to consider
> replacing target when reallocating the blockgroups.
> 
> This patch as such does not change any of the logic which is already in
> the original code, but instead it would bring such a logic at two
> locations into a single place/function. But to do that as these two
> location uses either WARN_ON or BUG_ON to perform the safety catch so
> the first patch does the cleanup and uses ASSERT at both of these
> places. And now since these two sections of codes are identical, the 2nd
> patch replaces them into a helper function.
> 
> Also though the check for num_device > 0 should actually be num_device >
> 1 OR instead, the check of num_device > 0 should have been after the
> num_device--,

I updated the patch following that logic now.

> since we have had already too many comments and added
> confusions, I shall leave that part to be fixed at sometime later. So
> that the cleanup here is a really a cleanup instead of chaning the
> logic in a rough scale.
> 
> Last but not least, this patch has gone through a lot of comments on
> either to use BUG or return -EINVAL for accidental fall of num_device
> below 1, which I wasn't expecting so much of the concerns about
> the use of BUG_ON/-EINVAL as its trivial because fundamentally a FS
> can't be in the mounted state when num_device < 1.
> 
> 
> Anand Jain (3):
>   btrfs: drop uuid_mutex in btrfs_free_extra_devids()
>   btrfs: assert for num_devices below 0
>   btrfs: add helper btrfs_num_devices() to deduce num_devices

Patches 2 and 3 are applied, with some adjustments. The first patch
needs another round of review as there were many changes regarding the
device locking and uuid_mutex. I need a break from this code as the
wounds haven't healed yet, but will add the patch to a topic branch to
next so I don't forget about it.

Re: [PATCH 1/2] btrfs: assert for num_devices below 0

[Anand Jain]

On 08/15/2018 08:11 PM, David Sterba wrote:
> On Fri, Aug 10, 2018 at 01:53:20PM +0800, Anand Jain wrote:
>> In preparation to add helper function to deduce the num_devices with
>> replace running, use assert instead of bug_on and warn_on.
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>>   fs/btrfs/volumes.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index b6d9b6a6fba7..0062615a79be 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -1877,7 +1877,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
>>   	num_devices = fs_devices->num_devices;
>>   	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
>>   	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
>> -		WARN_ON(num_devices < 1);
>> +		ASSERT(num_devices > 0);
>>   		num_devices--;
> 
> I was about to merge the patch but this gave me another opportunity to
> look at the code: the assertion should check for > 1. The value 1 of
> num_devices is sligthly wrong here and would lead to 0 returned from the
> function.

  Agree its slightly wrong but I kept it as it is to match with the
  original code. I mentioned about it in the cover letter.

Thanks, Anand

Re: [PATCH v2 0/3] Misc volume patch set part2

[Anand Jain]

On 08/15/2018 08:20 PM, David Sterba wrote:
> On Fri, Aug 10, 2018 at 01:53:18PM +0800, Anand Jain wrote:
>> The patch set (2/3 and 3/3) adds helper function to deduce the num_device
>> (which is the number of the devices when device is mounted, this excludes
>> the seed device however includes the replacing target). We need to know
>> the num_device that actually belongs to the FSID without the replacing
>> target (if it exists) when we are balancing and or when we are trying to
>> device delete. In both of these cases, it would be wrong to consider
>> replacing target when reallocating the blockgroups.
>>
>> This patch as such does not change any of the logic which is already in
>> the original code, but instead it would bring such a logic at two
>> locations into a single place/function. But to do that as these two
>> location uses either WARN_ON or BUG_ON to perform the safety catch so
>> the first patch does the cleanup and uses ASSERT at both of these
>> places. And now since these two sections of codes are identical, the 2nd
>> patch replaces them into a helper function.
>>
>> Also though the check for num_device > 0 should actually be num_device >
>> 1 OR instead, the check of num_device > 0 should have been after the
>> num_device--,
> 
> I updated the patch following that logic now.

  Right thanks.

>> since we have had already too many comments and added
>> confusions, I shall leave that part to be fixed at sometime later. So
>> that the cleanup here is a really a cleanup instead of chaning the
>> logic in a rough scale.
>>
>> Last but not least, this patch has gone through a lot of comments on
>> either to use BUG or return -EINVAL for accidental fall of num_device
>> below 1, which I wasn't expecting so much of the concerns about
>> the use of BUG_ON/-EINVAL as its trivial because fundamentally a FS
>> can't be in the mounted state when num_device < 1.
>>
>>
>> Anand Jain (3):
>>    btrfs: drop uuid_mutex in btrfs_free_extra_devids()
>>    btrfs: assert for num_devices below 0
>>    btrfs: add helper btrfs_num_devices() to deduce num_devices
> 
> Patches 2 and 3 are applied, with some adjustments.

  Adjustments at [1] are correct.

  [1]
   gitlab.com/kdave/btrfs-devel.git misc-next

Thanks, Anand.

> The first patch
> needs another round of review as there were many changes regarding the
> device locking and uuid_mutex. I need a break from this code as the
> wounds haven't healed yet, but will add the patch to a topic branch to
> next so I don't forget about it.