Re: [PATCH V2.1] btrfs-progs: do not merge tree block refs have different root_id

[Qu Wenruo]

On 2018年04月25日 13:19, Su Yue wrote:
> For an extent item which contains many tree block backrefs, like
> =
> In 020-extent-ref-cases/keyed_block_ref.img
> 
> item 10 key (29470720 METADATA_ITEM 0) itemoff 3450 itemsize 222
> refs 23 gen 10 flags TREE_BLOCK
> tree block skinny level 0
> tree block backref root 278
> tree block backref root 277
> tree block backref root 276
> tree block backref root 275
> tree block backref root 274
> tree block backref root 273
> tree block backref root 272
> tree block backref root 271
> tree block backref root 270
> tree block backref root 269
> tree block backref root 268
> tree block backref root 267
> tree block backref root 266
> tree block backref root 265
> tree block backref root 264
> tree block backref root 263
> tree block backref root 262
> tree block backref root 261
> tree block backref root 260
> tree block backref root 259
> tree block backref root 258
> tree block backref root 257
> =
> In find_parent_nodes(), these refs's parents are 0, then __merge_refs
> will merge refs to one ref. It causes only one root to be returned.
> 
> So, if both parents are 0, do not merge refs.
> 
> Lowmem check calls find_parent_nodes frequently to decide whether
> check an extent buffer or not. The bug influences bytes accounting.
> 
> Signed-off-by: Su Yue 

Reviewed-by: Qu Wenruo 

Waiting for the kernel port of backref.c to solve the problem permanently.

Thanks,
Qu

> ---
> Changelog:
> v2:
>   Put judgment of ref->parent above comparison.
>   Add the comment.
>   Fix typos.
> v2.1:
>   Remove the change of adding a new line.
> ---
>  backref.c | 6 ++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/backref.c b/backref.c
> index 51553c702187..23a394edfd02 100644
> --- a/backref.c
> +++ b/backref.c
> @@ -505,6 +505,12 @@ static void __merge_refs(struct pref_state *prefstate, 
> int mode)
>   if (!ref_for_same_block(ref1, ref2))
>   continue;
>   } else {
> + /*
> +  * Parent == 0 means that the ref is tree block
> +  * backref or its parent is unresolved.
> +  */
> + if (!ref1->parent || !ref2->parent)
> + continue;
>   if (ref1->parent != ref2->parent)
>   continue;
>   }
> 



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[PATCH V2.1] btrfs-progs: do not merge tree block refs have different root_id

[Su Yue]

For an extent item which contains many tree block backrefs, like
=
In 020-extent-ref-cases/keyed_block_ref.img

item 10 key (29470720 METADATA_ITEM 0) itemoff 3450 itemsize 222
refs 23 gen 10 flags TREE_BLOCK
tree block skinny level 0
tree block backref root 278
tree block backref root 277
tree block backref root 276
tree block backref root 275
tree block backref root 274
tree block backref root 273
tree block backref root 272
tree block backref root 271
tree block backref root 270
tree block backref root 269
tree block backref root 268
tree block backref root 267
tree block backref root 266
tree block backref root 265
tree block backref root 264
tree block backref root 263
tree block backref root 262
tree block backref root 261
tree block backref root 260
tree block backref root 259
tree block backref root 258
tree block backref root 257
=
In find_parent_nodes(), these refs's parents are 0, then __merge_refs
will merge refs to one ref. It causes only one root to be returned.

So, if both parents are 0, do not merge refs.

Lowmem check calls find_parent_nodes frequently to decide whether
check an extent buffer or not. The bug influences bytes accounting.

Signed-off-by: Su Yue 
---
Changelog:
v2:
  Put judgment of ref->parent above comparison.
  Add the comment.
  Fix typos.
v2.1:
  Remove the change of adding a new line.
---
 backref.c | 6 ++
 1 file changed, 6 insertions(+)

diff --git a/backref.c b/backref.c
index 51553c702187..23a394edfd02 100644
--- a/backref.c
+++ b/backref.c
@@ -505,6 +505,12 @@ static void __merge_refs(struct pref_state *prefstate, int 
mode)
if (!ref_for_same_block(ref1, ref2))
continue;
} else {
+   /*
+* Parent == 0 means that the ref is tree block
+* backref or its parent is unresolved.
+*/
+   if (!ref1->parent || !ref2->parent)
+   continue;
if (ref1->parent != ref2->parent)
continue;
}
-- 
2.17.0



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