[PATCH 0/2] Fixup and optimization for write time tree checker

[PATCH 0/2] Fixup and optimization for write time tree checker

[Qu Wenruo]

This patchset can be fetched from github:
https://github.com/adam900710/linux/tree/tree_checker_testing

Which is based on misc-next, with the following commit as HEAD:
commit 56d46f96de92ec69963acb7b1d9aed83d2a56a7b (david/misc-next-with-write-checks, david/misc-next)
Author: Nikolay Borisov <nborisov@suse.com>
Date:   Wed Mar 27 14:24:18 2019 +0200

    btrfs: Switch btrfs_trim_free_extents to find_first_clear_extent_bit

These two patches changes the behavior of write time tree-checker, where
the initial patches didn't check the content of the leaf, leaving memory
bit flipping possible to sneak in.

This patchset also introduces a new optimization, where original empty
leaf owner check can be pretty expensive and cause false alerts for
write time tree checker.
With this new optimization, write time tree checker can reuse the
existing btrfs_check_leaf_full().

Qu Wenruo (2):
  btrfs: tree-checker: Remove comprehensive root owner check
  btrfs: Do mandatory tree block check before submitting bio

 fs/btrfs/disk-io.c      | 13 +++++++++++++
 fs/btrfs/tree-checker.c | 24 ------------------------
 2 files changed, 13 insertions(+), 24 deletions(-)

-- 
2.21.0

[PATCH 1/2] btrfs: tree-checker: Remove comprehensive root owner check

[Qu Wenruo]

Commit 1ba98d086fe3 ("Btrfs: detect corruption when non-root leaf has
zero item") introduced comprehensive root owner checker.

However it's pretty expensive tree search to locate the owner root,
especially when it get reused by mandatory read and write time
tree-checker.

This patch will remove that check, and completely rely on owner based
empty leaf check, which is much faster and still works fine for most
case.

And since we skip the old root owner check, now write time tree check
can be merged with btrfs_check_leaf_full().

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/tree-checker.c | 24 ------------------------
 1 file changed, 24 deletions(-)

diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
index d2c3c1f8870d..e325d4df5c23 100644
--- a/fs/btrfs/tree-checker.c
+++ b/fs/btrfs/tree-checker.c
@@ -838,7 +838,6 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
 	 */
 	if (nritems == 0 && !btrfs_header_flag(leaf, BTRFS_HEADER_FLAG_RELOC)) {
 		u64 owner = btrfs_header_owner(leaf);
-		struct btrfs_root *check_root;
 
 		/* These trees must never be empty */
 		if (owner == BTRFS_ROOT_TREE_OBJECTID ||
@@ -852,29 +851,6 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
 				    owner);
 			return -EUCLEAN;
 		}
-		key.objectid = owner;
-		key.type = BTRFS_ROOT_ITEM_KEY;
-		key.offset = (u64)-1;
-
-		check_root = btrfs_get_fs_root(fs_info, &key, false);
-		/*
-		 * The only reason we also check NULL here is that during
-		 * open_ctree() some roots has not yet been set up.
-		 */
-		if (!IS_ERR_OR_NULL(check_root)) {
-			struct extent_buffer *eb;
-
-			eb = btrfs_root_node(check_root);
-			/* if leaf is the root, then it's fine */
-			if (leaf != eb) {
-				generic_err(leaf, 0,
-		"invalid nritems, have %u should not be 0 for non-root leaf",
-					nritems);
-				free_extent_buffer(eb);
-				return -EUCLEAN;
-			}
-			free_extent_buffer(eb);
-		}
 		return 0;
 	}
 
-- 
2.21.0

[PATCH 2/2] btrfs: Do mandatory tree block check before submitting bio

[Qu Wenruo]

There are at least 2 reports about a memory bit flip sneaking into
on-disk data.

Currently we only have a relaxed check triggered at
btrfs_mark_buffer_dirty() time, as it's not mandatory and only for
CONFIG_BTRFS_FS_CHECK_INTEGRITY enabled build, it doesn't help users to
detect such problem.

This patch will address the hole by triggering comprehensive check on
tree blocks before writing it back to disk.

The design points are:

- Timing of the check: Tree block write hook
  This timing is chosen to reduce the overhead.
  The comprehensive check should be as expensive as a checksum
  calculation.
  Doing full check at btrfs_mark_buffer_dirty() is too expensive for end
  user.

- Loose empty leaf check
  Originally for an empty leaf, tree-checker will report error if it's
  not a tree root.

  The problem for such check at write time is:
  * False alert for tree root created in current transaction
    In that case, the commit root still needs to be written to disk.
    And since current root can differ from commit root, then it will
    cause false alert.
    This happens for log tree.

  * False alert for relocated tree block
    Relocated tree block can be written to disk due to memory pressure,
    in that case an empty csum tree root can be written to disk and
    cause false alert, since csum root node hasn't been updated.

  Previous patch of removing comprehensive empty leaf owner check has
  paved the way for this patch.

The example error output will be something like:

  BTRFS critical (device dm-3): corrupt leaf: root=2 block=1350630375424 slot=68, bad key order, prev (10510212874240 169 0) current (1714119868416 169 0)
  BTRFS error (device dm-3): block=1350630375424 write time tree block corruption detected
  BTRFS: error (device dm-3) in btrfs_commit_transaction:2220: errno=-5 IO failure (Error while writing out transaction)
  BTRFS info (device dm-3): forced readonly
  BTRFS warning (device dm-3): Skipping commit of aborted transaction.
  BTRFS: error (device dm-3) in cleanup_transaction:1839: errno=-5 IO failure
  BTRFS info (device dm-3): delayed_refs has NO entry

Reported-by: Leonard Lausen <leonard@lausen.nl>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/disk-io.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 0b2b75a7efbd..c2c0640aea55 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -514,6 +514,7 @@ static int csum_dirty_buffer(struct btrfs_fs_info *fs_info, struct page *page)
 	u8 result[BTRFS_CSUM_SIZE];
 	u16 csum_size = btrfs_super_csum_size(fs_info->super_copy);
 	struct extent_buffer *eb;
+	int err;
 
 	eb = (struct extent_buffer *)page->private;
 	if (page != eb->pages[0])
@@ -535,7 +536,19 @@ static int csum_dirty_buffer(struct btrfs_fs_info *fs_info, struct page *page)
 	if (csum_tree_block(eb, result))
 		return -EINVAL;
 
+	if (btrfs_header_level(eb))
+		err = btrfs_check_node(fs_info, eb);
+	else
+		err = btrfs_check_leaf_full(fs_info, eb);
+
+	if (err < 0) {
+		btrfs_err(fs_info,
+		"block=%llu write time tree block corruption detected",
+			  eb->start);
+		return err;
+	}
 	write_extent_buffer(eb, result, 0, csum_size);
+
 	return 0;
 }
 
-- 
2.21.0

Re: [PATCH 1/2] btrfs: tree-checker: Remove comprehensive root owner check

[Nikolay Borisov]

On 4.04.19 г. 6:47 ч., Qu Wenruo wrote:
> Commit 1ba98d086fe3 ("Btrfs: detect corruption when non-root leaf has
> zero item") introduced comprehensive root owner checker.
> 
> However it's pretty expensive tree search to locate the owner root,
> especially when it get reused by mandatory read and write time
> tree-checker.
> 
> This patch will remove that check, and completely rely on owner based
> empty leaf check, which is much faster and still works fine for most
> case.
> 
> And since we skip the old root owner check, now write time tree check
> can be merged with btrfs_check_leaf_full().
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

This patch was sent yesterday, now it's sent again with no material
changes whatsoever? What's the point?

> ---
>  fs/btrfs/tree-checker.c | 24 ------------------------
>  1 file changed, 24 deletions(-)
> 
> diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
> index d2c3c1f8870d..e325d4df5c23 100644
> --- a/fs/btrfs/tree-checker.c
> +++ b/fs/btrfs/tree-checker.c
> @@ -838,7 +838,6 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
>  	 */
>  	if (nritems == 0 && !btrfs_header_flag(leaf, BTRFS_HEADER_FLAG_RELOC)) {
>  		u64 owner = btrfs_header_owner(leaf);
> -		struct btrfs_root *check_root;
>  
>  		/* These trees must never be empty */
>  		if (owner == BTRFS_ROOT_TREE_OBJECTID ||
> @@ -852,29 +851,6 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
>  				    owner);
>  			return -EUCLEAN;
>  		}
> -		key.objectid = owner;
> -		key.type = BTRFS_ROOT_ITEM_KEY;
> -		key.offset = (u64)-1;
> -
> -		check_root = btrfs_get_fs_root(fs_info, &key, false);
> -		/*
> -		 * The only reason we also check NULL here is that during
> -		 * open_ctree() some roots has not yet been set up.
> -		 */
> -		if (!IS_ERR_OR_NULL(check_root)) {
> -			struct extent_buffer *eb;
> -
> -			eb = btrfs_root_node(check_root);
> -			/* if leaf is the root, then it's fine */
> -			if (leaf != eb) {
> -				generic_err(leaf, 0,
> -		"invalid nritems, have %u should not be 0 for non-root leaf",
> -					nritems);
> -				free_extent_buffer(eb);
> -				return -EUCLEAN;
> -			}
> -			free_extent_buffer(eb);
> -		}
>  		return 0;
>  	}
>  
> 

Re: [PATCH 1/2] btrfs: tree-checker: Remove comprehensive root owner check

[Qu Wenruo]

On 2019/4/4 下午2:23, Nikolay Borisov wrote:
>
>
> On 4.04.19 г. 6:47 ч., Qu Wenruo wrote:
>> Commit 1ba98d086fe3 ("Btrfs: detect corruption when non-root leaf has
>> zero item") introduced comprehensive root owner checker.
>>
>> However it's pretty expensive tree search to locate the owner root,
>> especially when it get reused by mandatory read and write time
>> tree-checker.
>>
>> This patch will remove that check, and completely rely on owner based
>> empty leaf check, which is much faster and still works fine for most
>> case.
>>
>> And since we skip the old root owner check, now write time tree check
>> can be merged with btrfs_check_leaf_full().
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>
> This patch was sent yesterday, now it's sent again with no material
> changes whatsoever? What's the point?

This is the version to be applied on misc-next.

Thanks,
Qu

>
>> ---
>>  fs/btrfs/tree-checker.c | 24 ------------------------
>>  1 file changed, 24 deletions(-)
>>
>> diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
>> index d2c3c1f8870d..e325d4df5c23 100644
>> --- a/fs/btrfs/tree-checker.c
>> +++ b/fs/btrfs/tree-checker.c
>> @@ -838,7 +838,6 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
>>  	 */
>>  	if (nritems == 0 && !btrfs_header_flag(leaf, BTRFS_HEADER_FLAG_RELOC)) {
>>  		u64 owner = btrfs_header_owner(leaf);
>> -		struct btrfs_root *check_root;
>>
>>  		/* These trees must never be empty */
>>  		if (owner == BTRFS_ROOT_TREE_OBJECTID ||
>> @@ -852,29 +851,6 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
>>  				    owner);
>>  			return -EUCLEAN;
>>  		}
>> -		key.objectid = owner;
>> -		key.type = BTRFS_ROOT_ITEM_KEY;
>> -		key.offset = (u64)-1;
>> -
>> -		check_root = btrfs_get_fs_root(fs_info, &key, false);
>> -		/*
>> -		 * The only reason we also check NULL here is that during
>> -		 * open_ctree() some roots has not yet been set up.
>> -		 */
>> -		if (!IS_ERR_OR_NULL(check_root)) {
>> -			struct extent_buffer *eb;
>> -
>> -			eb = btrfs_root_node(check_root);
>> -			/* if leaf is the root, then it's fine */
>> -			if (leaf != eb) {
>> -				generic_err(leaf, 0,
>> -		"invalid nritems, have %u should not be 0 for non-root leaf",
>> -					nritems);
>> -				free_extent_buffer(eb);
>> -				return -EUCLEAN;
>> -			}
>> -			free_extent_buffer(eb);
>> -		}
>>  		return 0;
>>  	}
>>
>>

Re: [PATCH 1/2] btrfs: tree-checker: Remove comprehensive root owner check

[David Sterba]

On Thu, Apr 04, 2019 at 09:23:11AM +0300, Nikolay Borisov wrote:
> 
> 
> On 4.04.19 г. 6:47 ч., Qu Wenruo wrote:
> > Commit 1ba98d086fe3 ("Btrfs: detect corruption when non-root leaf has
> > zero item") introduced comprehensive root owner checker.
> > 
> > However it's pretty expensive tree search to locate the owner root,
> > especially when it get reused by mandatory read and write time
> > tree-checker.
> > 
> > This patch will remove that check, and completely rely on owner based
> > empty leaf check, which is much faster and still works fine for most
> > case.
> > 
> > And since we skip the old root owner check, now write time tree check
> > can be merged with btrfs_check_leaf_full().
> > 
> > Signed-off-by: Qu Wenruo <wqu@suse.com>
> 
> This patch was sent yesterday, now it's sent again with no material
> changes whatsoever? What's the point?

The core change is the same, but there are removed hunks compared to v1.
We've discussed that yesterday and Qu did it right.

Re: [PATCH 0/2] Fixup and optimization for write time tree checker

[David Sterba]

On Thu, Apr 04, 2019 at 11:47:06AM +0800, Qu Wenruo wrote:
> This patchset can be fetched from github:
> https://github.com/adam900710/linux/tree/tree_checker_testing
> 
> Which is based on misc-next, with the following commit as HEAD:
> commit 56d46f96de92ec69963acb7b1d9aed83d2a56a7b (david/misc-next-with-write-checks, david/misc-next)
> Author: Nikolay Borisov <nborisov@suse.com>
> Date:   Wed Mar 27 14:24:18 2019 +0200
> 
>     btrfs: Switch btrfs_trim_free_extents to find_first_clear_extent_bit
> 
> These two patches changes the behavior of write time tree-checker, where
> the initial patches didn't check the content of the leaf, leaving memory
> bit flipping possible to sneak in.
> 
> This patchset also introduces a new optimization, where original empty
> leaf owner check can be pretty expensive and cause false alerts for
> write time tree checker.
> With this new optimization, write time tree checker can reuse the
> existing btrfs_check_leaf_full().

This on top of misc-next throws 6 'write time' errors:

btrfs/101               [14:00:24][ 3963.736352] run fstests btrfs/101 at 2019-04-05 14:00:24
[ 3970.417649] BTRFS error (device vdc): block=2446344192 write time tree block corruption detected

btrfs/151               [14:06:52][ 4351.133395] run fstests btrfs/151 at 2019-04-05 14:06:52
[ 4351.659771] BTRFS error (device vdb): block=570572800 write time tree block corruption detected

btrfs/164               [14:07:19][ 4378.349990] run fstests btrfs/164 at 2019-04-05 14:07:19
[ 4380.645918] BTRFS error (device vdb): block=2479882240 write time tree block corruption detected

btrfs/176               [14:07:39][ 4398.493736] run fstests btrfs/176 at 2019-04-05 14:07:39
[ 4399.186203] BTRFS error (device vdb): block=22036480 write time tree block corruption detected
[ 4399.302700] BTRFS error (device vdb): block=1372585984 write time tree block corruption detected

btrfs/184               [14:10:13][ 4552.496289] run fstests btrfs/184 at 2019-04-05 14:10:13
[ 4553.292780] BTRFS error (device vdb): block=4602216448 write time tree block corruption detected

Have you seen the errors during your testing?

Re: [PATCH 0/2] Fixup and optimization for write time tree checker

[Qu Wenruo]

[-- Attachment #1.1: Type: text/plain, Size: 2733 bytes --]



On 2019/4/5 下午11:49, David Sterba wrote:
> On Thu, Apr 04, 2019 at 11:47:06AM +0800, Qu Wenruo wrote:
>> This patchset can be fetched from github:
>> https://github.com/adam900710/linux/tree/tree_checker_testing
>>
>> Which is based on misc-next, with the following commit as HEAD:
>> commit 56d46f96de92ec69963acb7b1d9aed83d2a56a7b (david/misc-next-with-write-checks, david/misc-next)
>> Author: Nikolay Borisov <nborisov@suse.com>
>> Date:   Wed Mar 27 14:24:18 2019 +0200
>>
>>     btrfs: Switch btrfs_trim_free_extents to find_first_clear_extent_bit
>>
>> These two patches changes the behavior of write time tree-checker, where
>> the initial patches didn't check the content of the leaf, leaving memory
>> bit flipping possible to sneak in.
>>
>> This patchset also introduces a new optimization, where original empty
>> leaf owner check can be pretty expensive and cause false alerts for
>> write time tree checker.
>> With this new optimization, write time tree checker can reuse the
>> existing btrfs_check_leaf_full().
> 
> This on top of misc-next throws 6 'write time' errors:
> 
> btrfs/101               [14:00:24][ 3963.736352] run fstests btrfs/101 at 2019-04-05 14:00:24
> [ 3970.417649] BTRFS error (device vdc): block=2446344192 write time tree block corruption detected

In my case, these tests just pass, so I haven't checked the dmesg.

But indeed I can reproduce the same dmesg line.

Before removing the device, btrfs will use btrfs_shrink_device(new_size
= 0) to remove all data from that device.

And that will cause on-disk total_bytes to be 0, triggering the bug.

So indeed the behavior is different between read and write time.

I'll check how to do it correctly.

Thanks,
Qu

> 
> btrfs/151               [14:06:52][ 4351.133395] run fstests btrfs/151 at 2019-04-05 14:06:52
> [ 4351.659771] BTRFS error (device vdb): block=570572800 write time tree block corruption detected
> 
> btrfs/164               [14:07:19][ 4378.349990] run fstests btrfs/164 at 2019-04-05 14:07:19
> [ 4380.645918] BTRFS error (device vdb): block=2479882240 write time tree block corruption detected
> 
> btrfs/176               [14:07:39][ 4398.493736] run fstests btrfs/176 at 2019-04-05 14:07:39
> [ 4399.186203] BTRFS error (device vdb): block=22036480 write time tree block corruption detected
> [ 4399.302700] BTRFS error (device vdb): block=1372585984 write time tree block corruption detected
> 
> btrfs/184               [14:10:13][ 4552.496289] run fstests btrfs/184 at 2019-04-05 14:10:13
> [ 4553.292780] BTRFS error (device vdb): block=4602216448 write time tree block corruption detected
> 
> Have you seen the errors during your testing?
> 


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Re: [PATCH 0/2] Fixup and optimization for write time tree checker

[Qu Wenruo]

[-- Attachment #1.1: Type: text/plain, Size: 3164 bytes --]



On 2019/4/6 上午8:22, Qu Wenruo wrote:
> 
> 
> On 2019/4/5 下午11:49, David Sterba wrote:
>> On Thu, Apr 04, 2019 at 11:47:06AM +0800, Qu Wenruo wrote:
>>> This patchset can be fetched from github:
>>> https://github.com/adam900710/linux/tree/tree_checker_testing
>>>
>>> Which is based on misc-next, with the following commit as HEAD:
>>> commit 56d46f96de92ec69963acb7b1d9aed83d2a56a7b (david/misc-next-with-write-checks, david/misc-next)
>>> Author: Nikolay Borisov <nborisov@suse.com>
>>> Date:   Wed Mar 27 14:24:18 2019 +0200
>>>
>>>     btrfs: Switch btrfs_trim_free_extents to find_first_clear_extent_bit
>>>
>>> These two patches changes the behavior of write time tree-checker, where
>>> the initial patches didn't check the content of the leaf, leaving memory
>>> bit flipping possible to sneak in.
>>>
>>> This patchset also introduces a new optimization, where original empty
>>> leaf owner check can be pretty expensive and cause false alerts for
>>> write time tree checker.
>>> With this new optimization, write time tree checker can reuse the
>>> existing btrfs_check_leaf_full().
>>
>> This on top of misc-next throws 6 'write time' errors:
>>
>> btrfs/101               [14:00:24][ 3963.736352] run fstests btrfs/101 at 2019-04-05 14:00:24
>> [ 3970.417649] BTRFS error (device vdc): block=2446344192 write time tree block corruption detected
> 
> In my case, these tests just pass, so I haven't checked the dmesg.

Oh, those tests passes because of a bug in mainline.

6dc4f100c175 ("block: allow bio_for_each_segment_all() to iterate over
multi-page bvec") prevents us from breaking bio_for_each_segment_all()
loop, thus not triggering transaction abort.

A nice finding digging into the report.

Thanks,
Qu

> 
> But indeed I can reproduce the same dmesg line.
> 
> Before removing the device, btrfs will use btrfs_shrink_device(new_size
> = 0) to remove all data from that device.
> 
> And that will cause on-disk total_bytes to be 0, triggering the bug.
> 
> So indeed the behavior is different between read and write time.
> 
> I'll check how to do it correctly.
> 
> Thanks,
> Qu
> 
>>
>> btrfs/151               [14:06:52][ 4351.133395] run fstests btrfs/151 at 2019-04-05 14:06:52
>> [ 4351.659771] BTRFS error (device vdb): block=570572800 write time tree block corruption detected
>>
>> btrfs/164               [14:07:19][ 4378.349990] run fstests btrfs/164 at 2019-04-05 14:07:19
>> [ 4380.645918] BTRFS error (device vdb): block=2479882240 write time tree block corruption detected
>>
>> btrfs/176               [14:07:39][ 4398.493736] run fstests btrfs/176 at 2019-04-05 14:07:39
>> [ 4399.186203] BTRFS error (device vdb): block=22036480 write time tree block corruption detected
>> [ 4399.302700] BTRFS error (device vdb): block=1372585984 write time tree block corruption detected
>>
>> btrfs/184               [14:10:13][ 4552.496289] run fstests btrfs/184 at 2019-04-05 14:10:13
>> [ 4553.292780] BTRFS error (device vdb): block=4602216448 write time tree block corruption detected
>>
>> Have you seen the errors during your testing?
>>
> 


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Re: [PATCH 0/2] Fixup and optimization for write time tree checker

[David Sterba]

On Sat, Apr 06, 2019 at 09:57:35AM +0800, Qu Wenruo wrote:
> >>>
> >>> This patchset also introduces a new optimization, where original empty
> >>> leaf owner check can be pretty expensive and cause false alerts for
> >>> write time tree checker.
> >>> With this new optimization, write time tree checker can reuse the
> >>> existing btrfs_check_leaf_full().
> >>
> >> This on top of misc-next throws 6 'write time' errors:
> >>
> >> btrfs/101               [14:00:24][ 3963.736352] run fstests btrfs/101 at 2019-04-05 14:00:24
> >> [ 3970.417649] BTRFS error (device vdc): block=2446344192 write time tree block corruption detected
> > 
> > In my case, these tests just pass, so I haven't checked the dmesg.

I have a filter for log (it's mix of terminal and kernel log)

egrep -a '(run fstests|WARNING:|BUG:|RIP:|UBSAN:|^\s+[+-]|CPU:.*Comm:|write time|read time|blocked for more than|Linux version)'

so the unexpected output is not lost among other messages.

> Oh, those tests passes because of a bug in mainline.
> 
> 6dc4f100c175 ("block: allow bio_for_each_segment_all() to iterate over
> multi-page bvec") prevents us from breaking bio_for_each_segment_all()
> loop, thus not triggering transaction abort.
> 
> A nice finding digging into the report.

Indeed, thanks. The hidden for-in-for in the macro would bite us later.

Re: [PATCH 2/2] btrfs: Do mandatory tree block check before submitting bio

[David Sterba]

On Thu, Apr 04, 2019 at 11:47:08AM +0800, Qu Wenruo wrote:
> @@ -514,6 +514,7 @@ static int csum_dirty_buffer(struct btrfs_fs_info *fs_info, struct page *page)
>  	u8 result[BTRFS_CSUM_SIZE];
>  	u16 csum_size = btrfs_super_csum_size(fs_info->super_copy);
>  	struct extent_buffer *eb;
> +	int err;

Please next time user 'ret' for newly added return values, I've fixed
that in the commit.