[PATCH 1/2] btrfs: include the free space tree in the global rsv minimum calculation

[PATCH 1/2] btrfs: include the free space tree in the global rsv minimum calculation

[Josef Bacik]

Filipe reported a problem where generic/619 was failing with an ENOSPC
abort while running delayed refs, like the following

------------[ cut here ]------------
BTRFS: Transaction aborted (error -28)
WARNING: CPU: 3 PID: 522920 at fs/btrfs/free-space-tree.c:1049 add_to_free_space_tree+0xe5/0x110 [btrfs]
CPU: 3 PID: 522920 Comm: kworker/u16:19 Tainted: G        W         5.16.0-rc2-btrfs-next-106 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
Workqueue: events_unbound btrfs_async_reclaim_metadata_space [btrfs]
RIP: 0010:add_to_free_space_tree+0xe5/0x110 [btrfs]
RSP: 0000:ffffa65087fb7b20 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffffffff9131eeaa RDI: 00000000ffffffff
RBP: ffff8d62e26481b8 R08: ffffffff9ad97ce0 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000001 R12: 00000000ffffffe4
R13: ffff8d61c25fe688 R14: ffff8d61ebd88800 R15: ffff8d61ebd88a90
FS:  0000000000000000(0000) GS:ffff8d64ed400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa46a8b1000 CR3: 0000000148d18003 CR4: 0000000000370ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __btrfs_free_extent+0x516/0x950 [btrfs]
 __btrfs_run_delayed_refs+0x2b1/0x1250 [btrfs]
 btrfs_run_delayed_refs+0x86/0x210 [btrfs]
 flush_space+0x403/0x630 [btrfs]
 ? call_rcu_tasks_generic+0x50/0x80
 ? lock_release+0x223/0x4a0
 ? btrfs_get_alloc_profile+0xb5/0x290 [btrfs]
 ? do_raw_spin_unlock+0x4b/0xa0
 btrfs_async_reclaim_metadata_space+0x139/0x320 [btrfs]
 process_one_work+0x24c/0x5b0
 worker_thread+0x55/0x3c0
 ? process_one_work+0x5b0/0x5b0
 kthread+0x17c/0x1a0
 ? set_kthread_struct+0x40/0x40
 ret_from_fork+0x22/0x30

There's a couple of reasons for this, but in generic/619's case the
largest reason is because it is a very small file system, ad we do not
reserve enough space for the global reserve.

With the free space tree we now have the free space tree that we need to
modify when running delayed refs.  This means we need the global reserve
to take this into account when it calculates the minimum size it needs
to be.  This is especially important for very small file systems.

Fix this by adjusting the minimum global block rsv size math to include
the size of the free space tree when calculating the size.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/block-rsv.c | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index 21ac60ec19f6..b3086f252ad0 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -352,25 +352,29 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
 	struct btrfs_space_info *sinfo = block_rsv->space_info;
-	struct btrfs_root *extent_root = btrfs_extent_root(fs_info, 0);
-	struct btrfs_root *csum_root = btrfs_csum_root(fs_info, 0);
-	u64 num_bytes;
-	unsigned min_items;
+	struct btrfs_root *root, *tmp;
+	u64 num_bytes = btrfs_root_used(&fs_info->tree_root->root_item);
+	unsigned int min_items = 1;
 
 	/*
 	 * The global block rsv is based on the size of the extent tree, the
 	 * checksum tree and the root tree.  If the fs is empty we want to set
 	 * it to a minimal amount for safety.
+	 *
+	 * We also are going to need to modify the minimum of the tree root and
+	 * any global roots we could touch.
 	 */
-	num_bytes = btrfs_root_used(&extent_root->root_item) +
-		btrfs_root_used(&csum_root->root_item) +
-		btrfs_root_used(&fs_info->tree_root->root_item);
-
-	/*
-	 * We at a minimum are going to modify the csum root, the tree root, and
-	 * the extent root.
-	 */
-	min_items = 3;
+	read_lock(&fs_info->global_root_lock);
+	rbtree_postorder_for_each_entry_safe(root, tmp, &fs_info->global_root_tree,
+					     rb_node) {
+		if (root->root_key.objectid == BTRFS_EXTENT_TREE_OBJECTID ||
+		    root->root_key.objectid == BTRFS_CSUM_TREE_OBJECTID ||
+		    root->root_key.objectid == BTRFS_FREE_SPACE_TREE_OBJECTID) {
+			num_bytes += btrfs_root_used(&root->root_item);
+			min_items++;
+		}
+	}
+	read_unlock(&fs_info->global_root_lock);
 
 	/*
 	 * But we also want to reserve enough space so we can do the fallback
-- 
2.26.3

[PATCH 2/2] btrfs: reserve extra space for the free space tree

[Josef Bacik]

Filipe reported a problem where sometimes he'd get an ENOSPC abort when
running delayed refs with generic/619 and the free space tree enabled.
This is partly because we do not reserve space for modifying the free
space tree, nor do we have a block rsv associated with that tree.  Fix
this by making sure any free space tree defaults to using the
delayed_refs_rsv, and make sure we reserve the space for those
allocations.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/block-rsv.c   |  1 +
 fs/btrfs/delayed-ref.c | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index b3086f252ad0..b3ee49b0b1e8 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -426,6 +426,7 @@ void btrfs_init_root_block_rsv(struct btrfs_root *root)
 	switch (root->root_key.objectid) {
 	case BTRFS_CSUM_TREE_OBJECTID:
 	case BTRFS_EXTENT_TREE_OBJECTID:
+	case BTRFS_FREE_SPACE_TREE_OBJECTID:
 		root->block_rsv = &fs_info->delayed_refs_rsv;
 		break;
 	case BTRFS_ROOT_TREE_OBJECTID:
diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index da9d20813147..533521be8fdf 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -84,6 +84,17 @@ void btrfs_delayed_refs_rsv_release(struct btrfs_fs_info *fs_info, int nr)
 	u64 num_bytes = btrfs_calc_insert_metadata_size(fs_info, nr);
 	u64 released = 0;
 
+	/*
+	 * We have to check the mount option here because we could be enabling
+	 * the free space tree for the first time and don't have the compat_ro
+	 * option set yet.
+	 *
+	 * We need extra reservations if we have the free space tree because
+	 * we'll have to modify that tree as well.
+	 */
+	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE))
+		num_bytes <<= 1;
+
 	released = btrfs_block_rsv_release(fs_info, block_rsv, num_bytes, NULL);
 	if (released)
 		trace_btrfs_space_reservation(fs_info, "delayed_refs_rsv",
@@ -108,6 +119,17 @@ void btrfs_update_delayed_refs_rsv(struct btrfs_trans_handle *trans)
 
 	num_bytes = btrfs_calc_insert_metadata_size(fs_info,
 						    trans->delayed_ref_updates);
+	/*
+	 * We have to check the mount option here because we could be enabling
+	 * the free space tree for the first time and don't have the compat_ro
+	 * option set yet.
+	 *
+	 * We need extra reservations if we have the free space tree because
+	 * we'll have to modify that tree as well.
+	 */
+	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE))
+		num_bytes <<= 1;
+
 	spin_lock(&delayed_rsv->lock);
 	delayed_rsv->size += num_bytes;
 	delayed_rsv->full = 0;
-- 
2.26.3

Re: [PATCH 1/2] btrfs: include the free space tree in the global rsv minimum calculation

[Nikolay Borisov]

On 1.12.21 г. 18:45, Josef Bacik wrote:
> Filipe reported a problem where generic/619 was failing with an ENOSPC
> abort while running delayed refs, like the following
> 
> ------------[ cut here ]------------
> BTRFS: Transaction aborted (error -28)
> WARNING: CPU: 3 PID: 522920 at fs/btrfs/free-space-tree.c:1049 add_to_free_space_tree+0xe5/0x110 [btrfs]
> CPU: 3 PID: 522920 Comm: kworker/u16:19 Tainted: G        W         5.16.0-rc2-btrfs-next-106 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> Workqueue: events_unbound btrfs_async_reclaim_metadata_space [btrfs]
> RIP: 0010:add_to_free_space_tree+0xe5/0x110 [btrfs]
> RSP: 0000:ffffa65087fb7b20 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000000
> RDX: 0000000000000001 RSI: ffffffff9131eeaa RDI: 00000000ffffffff
> RBP: ffff8d62e26481b8 R08: ffffffff9ad97ce0 R09: 0000000000000001
> R10: 0000000000000000 R11: 0000000000000001 R12: 00000000ffffffe4
> R13: ffff8d61c25fe688 R14: ffff8d61ebd88800 R15: ffff8d61ebd88a90
> FS:  0000000000000000(0000) GS:ffff8d64ed400000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fa46a8b1000 CR3: 0000000148d18003 CR4: 0000000000370ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  __btrfs_free_extent+0x516/0x950 [btrfs]
>  __btrfs_run_delayed_refs+0x2b1/0x1250 [btrfs]
>  btrfs_run_delayed_refs+0x86/0x210 [btrfs]
>  flush_space+0x403/0x630 [btrfs]
>  ? call_rcu_tasks_generic+0x50/0x80
>  ? lock_release+0x223/0x4a0
>  ? btrfs_get_alloc_profile+0xb5/0x290 [btrfs]
>  ? do_raw_spin_unlock+0x4b/0xa0
>  btrfs_async_reclaim_metadata_space+0x139/0x320 [btrfs]
>  process_one_work+0x24c/0x5b0
>  worker_thread+0x55/0x3c0
>  ? process_one_work+0x5b0/0x5b0
>  kthread+0x17c/0x1a0
>  ? set_kthread_struct+0x40/0x40
>  ret_from_fork+0x22/0x30
> 
> There's a couple of reasons for this, but in generic/619's case the
> largest reason is because it is a very small file system, ad we do not
> reserve enough space for the global reserve.
> 
> With the free space tree we now have the free space tree that we need to
> modify when running delayed refs.  This means we need the global reserve
> to take this into account when it calculates the minimum size it needs
> to be.  This is especially important for very small file systems.
> 
> Fix this by adjusting the minimum global block rsv size math to include
> the size of the free space tree when calculating the size.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/block-rsv.c | 30 +++++++++++++++++-------------
>  1 file changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> index 21ac60ec19f6..b3086f252ad0 100644
> --- a/fs/btrfs/block-rsv.c
> +++ b/fs/btrfs/block-rsv.c
> @@ -352,25 +352,29 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
>  {
>  	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
>  	struct btrfs_space_info *sinfo = block_rsv->space_info;
> -	struct btrfs_root *extent_root = btrfs_extent_root(fs_info, 0);
> -	struct btrfs_root *csum_root = btrfs_csum_root(fs_info, 0);
> -	u64 num_bytes;
> -	unsigned min_items;
> +	struct btrfs_root *root, *tmp;
> +	u64 num_bytes = btrfs_root_used(&fs_info->tree_root->root_item);
> +	unsigned int min_items = 1;
>  
>  	/*
>  	 * The global block rsv is based on the size of the extent tree, the
>  	 * checksum tree and the root tree.  If the fs is empty we want to set
>  	 * it to a minimal amount for safety.
> +	 *
> +	 * We also are going to need to modify the minimum of the tree root and
> +	 * any global roots we could touch.
>  	 */
> -	num_bytes = btrfs_root_used(&extent_root->root_item) +
> -		btrfs_root_used(&csum_root->root_item) +
> -		btrfs_root_used(&fs_info->tree_root->root_item);
> -
> -	/*
> -	 * We at a minimum are going to modify the csum root, the tree root, and
> -	 * the extent root.
> -	 */
> -	min_items = 3;
> +	read_lock(&fs_info->global_root_lock);
> +	rbtree_postorder_for_each_entry_safe(root, tmp, &fs_info->global_root_tree,
> +					     rb_node) {
> +		if (root->root_key.objectid == BTRFS_EXTENT_TREE_OBJECTID ||
> +		    root->root_key.objectid == BTRFS_CSUM_TREE_OBJECTID ||
> +		    root->root_key.objectid == BTRFS_FREE_SPACE_TREE_OBJECTID) {
> +			num_bytes += btrfs_root_used(&root->root_item);
> +			min_items++;
> +		}
> +	}

nit: I think those changes constitute 2 patches - the first does the
global_root_tree iteration as preparation for global roots. And the
subsequent patch should also include the freespace tree, no ? Otherwise
LGTM.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> +	read_unlock(&fs_info->global_root_lock);
>  
>  	/*
>  	 * But we also want to reserve enough space so we can do the fallback
> 

Re: [PATCH 2/2] btrfs: reserve extra space for the free space tree

[Nikolay Borisov]

On 1.12.21 г. 18:45, Josef Bacik wrote:
> Filipe reported a problem where sometimes he'd get an ENOSPC abort when
> running delayed refs with generic/619 and the free space tree enabled.
> This is partly because we do not reserve space for modifying the free
> space tree, nor do we have a block rsv associated with that tree.  Fix
> this by making sure any free space tree defaults to using the
> delayed_refs_rsv, and make sure we reserve the space for those
> allocations.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/block-rsv.c   |  1 +
>  fs/btrfs/delayed-ref.c | 22 ++++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> index b3086f252ad0..b3ee49b0b1e8 100644
> --- a/fs/btrfs/block-rsv.c
> +++ b/fs/btrfs/block-rsv.c
> @@ -426,6 +426,7 @@ void btrfs_init_root_block_rsv(struct btrfs_root *root)
>  	switch (root->root_key.objectid) {
>  	case BTRFS_CSUM_TREE_OBJECTID:
>  	case BTRFS_EXTENT_TREE_OBJECTID:
> +	case BTRFS_FREE_SPACE_TREE_OBJECTID:
>  		root->block_rsv = &fs_info->delayed_refs_rsv;
>  		break;
>  	case BTRFS_ROOT_TREE_OBJECTID:
> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
> index da9d20813147..533521be8fdf 100644
> --- a/fs/btrfs/delayed-ref.c
> +++ b/fs/btrfs/delayed-ref.c
> @@ -84,6 +84,17 @@ void btrfs_delayed_refs_rsv_release(struct btrfs_fs_info *fs_info, int nr)
>  	u64 num_bytes = btrfs_calc_insert_metadata_size(fs_info, nr);
>  	u64 released = 0;
>  
> +	/*
> +	 * We have to check the mount option here because we could be enabling
> +	 * the free space tree for the first time and don't have the compat_ro
> +	 * option set yet.
> +	 *
> +	 * We need extra reservations if we have the free space tree because
> +	 * we'll have to modify that tree as well.
> +	 */
> +	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE))
> +		num_bytes <<= 1;
> +
>  	released = btrfs_block_rsv_release(fs_info, block_rsv, num_bytes, NULL);
>  	if (released)
>  		trace_btrfs_space_reservation(fs_info, "delayed_refs_rsv",
> @@ -108,6 +119,17 @@ void btrfs_update_delayed_refs_rsv(struct btrfs_trans_handle *trans)
>  
>  	num_bytes = btrfs_calc_insert_metadata_size(fs_info,
>  						    trans->delayed_ref_updates);
> +	/*
> +	 * We have to check the mount option here because we could be enabling
> +	 * the free space tree for the first time and don't have the compat_ro
> +	 * option set yet.
> +	 *
> +	 * We need extra reservations if we have the free space tree because
> +	 * we'll have to modify that tree as well.
> +	 */
> +	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE))
> +		num_bytes <<= 1;

That num_bytes * 2 is heuristically derived, right? If so I'd like this
to be mentioned explicitly in the changelog.

> +
>  	spin_lock(&delayed_rsv->lock);
>  	delayed_rsv->size += num_bytes;
>  	delayed_rsv->full = 0;
> 

Re: [PATCH 2/2] btrfs: reserve extra space for the free space tree

[Josef Bacik]

On Thu, Dec 02, 2021 at 04:14:51PM +0200, Nikolay Borisov wrote:
> 
> 
> On 1.12.21 г. 18:45, Josef Bacik wrote:
> > Filipe reported a problem where sometimes he'd get an ENOSPC abort when
> > running delayed refs with generic/619 and the free space tree enabled.
> > This is partly because we do not reserve space for modifying the free
> > space tree, nor do we have a block rsv associated with that tree.  Fix
> > this by making sure any free space tree defaults to using the
> > delayed_refs_rsv, and make sure we reserve the space for those
> > allocations.
> > 
> > Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> > ---
> >  fs/btrfs/block-rsv.c   |  1 +
> >  fs/btrfs/delayed-ref.c | 22 ++++++++++++++++++++++
> >  2 files changed, 23 insertions(+)
> > 
> > diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> > index b3086f252ad0..b3ee49b0b1e8 100644
> > --- a/fs/btrfs/block-rsv.c
> > +++ b/fs/btrfs/block-rsv.c
> > @@ -426,6 +426,7 @@ void btrfs_init_root_block_rsv(struct btrfs_root *root)
> >  	switch (root->root_key.objectid) {
> >  	case BTRFS_CSUM_TREE_OBJECTID:
> >  	case BTRFS_EXTENT_TREE_OBJECTID:
> > +	case BTRFS_FREE_SPACE_TREE_OBJECTID:
> >  		root->block_rsv = &fs_info->delayed_refs_rsv;
> >  		break;
> >  	case BTRFS_ROOT_TREE_OBJECTID:
> > diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
> > index da9d20813147..533521be8fdf 100644
> > --- a/fs/btrfs/delayed-ref.c
> > +++ b/fs/btrfs/delayed-ref.c
> > @@ -84,6 +84,17 @@ void btrfs_delayed_refs_rsv_release(struct btrfs_fs_info *fs_info, int nr)
> >  	u64 num_bytes = btrfs_calc_insert_metadata_size(fs_info, nr);
> >  	u64 released = 0;
> >  
> > +	/*
> > +	 * We have to check the mount option here because we could be enabling
> > +	 * the free space tree for the first time and don't have the compat_ro
> > +	 * option set yet.
> > +	 *
> > +	 * We need extra reservations if we have the free space tree because
> > +	 * we'll have to modify that tree as well.
> > +	 */
> > +	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE))
> > +		num_bytes <<= 1;
> > +
> >  	released = btrfs_block_rsv_release(fs_info, block_rsv, num_bytes, NULL);
> >  	if (released)
> >  		trace_btrfs_space_reservation(fs_info, "delayed_refs_rsv",
> > @@ -108,6 +119,17 @@ void btrfs_update_delayed_refs_rsv(struct btrfs_trans_handle *trans)
> >  
> >  	num_bytes = btrfs_calc_insert_metadata_size(fs_info,
> >  						    trans->delayed_ref_updates);
> > +	/*
> > +	 * We have to check the mount option here because we could be enabling
> > +	 * the free space tree for the first time and don't have the compat_ro
> > +	 * option set yet.
> > +	 *
> > +	 * We need extra reservations if we have the free space tree because
> > +	 * we'll have to modify that tree as well.
> > +	 */
> > +	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE))
> > +		num_bytes <<= 1;
> 
> That num_bytes * 2 is heuristically derived, right? If so I'd like this
> to be mentioned explicitly in the changelog.

No, the delayed refs is number of items we're modifying in the extent tree.  If
we have the free space tree we have to modify the same number of items in the
free space tree, so it's 2x the space reservations required.  Thanks,

Josef

Re: [PATCH 1/2] btrfs: include the free space tree in the global rsv minimum calculation

[David Sterba]

On Thu, Dec 02, 2021 at 11:07:51AM +0200, Nikolay Borisov wrote:
> > diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> > index 21ac60ec19f6..b3086f252ad0 100644
> > --- a/fs/btrfs/block-rsv.c
> > +++ b/fs/btrfs/block-rsv.c
> > @@ -352,25 +352,29 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
> >  {
> >  	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
> >  	struct btrfs_space_info *sinfo = block_rsv->space_info;
> > -	struct btrfs_root *extent_root = btrfs_extent_root(fs_info, 0);
> > -	struct btrfs_root *csum_root = btrfs_csum_root(fs_info, 0);
> > -	u64 num_bytes;
> > -	unsigned min_items;
> > +	struct btrfs_root *root, *tmp;
> > +	u64 num_bytes = btrfs_root_used(&fs_info->tree_root->root_item);
> > +	unsigned int min_items = 1;
> >  
> >  	/*
> >  	 * The global block rsv is based on the size of the extent tree, the
> >  	 * checksum tree and the root tree.  If the fs is empty we want to set
> >  	 * it to a minimal amount for safety.
> > +	 *
> > +	 * We also are going to need to modify the minimum of the tree root and
> > +	 * any global roots we could touch.
> >  	 */
> > -	num_bytes = btrfs_root_used(&extent_root->root_item) +
> > -		btrfs_root_used(&csum_root->root_item) +
> > -		btrfs_root_used(&fs_info->tree_root->root_item);
> > -
> > -	/*
> > -	 * We at a minimum are going to modify the csum root, the tree root, and
> > -	 * the extent root.
> > -	 */
> > -	min_items = 3;
> > +	read_lock(&fs_info->global_root_lock);
> > +	rbtree_postorder_for_each_entry_safe(root, tmp, &fs_info->global_root_tree,
> > +					     rb_node) {
> > +		if (root->root_key.objectid == BTRFS_EXTENT_TREE_OBJECTID ||
> > +		    root->root_key.objectid == BTRFS_CSUM_TREE_OBJECTID ||
> > +		    root->root_key.objectid == BTRFS_FREE_SPACE_TREE_OBJECTID) {
> > +			num_bytes += btrfs_root_used(&root->root_item);
> > +			min_items++;
> > +		}
> > +	}
> 
> nit: I think those changes constitute 2 patches - the first does the
> global_root_tree iteration as preparation for global roots. And the
> subsequent patch should also include the freespace tree, no ? Otherwise
> LGTM.

As this is probably a fix we'd like to backport, the oder of the
suggested patches would have to be reversed too, so we first have the
fix and then port to the global roots. But the diff is short and a
backport to non-global roots could be done in one go as well,
referencing this patch.