[v2,1/3] btrfs: backref: Introduce the skeleton of btrfs_backref_iterator
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Message ID 20200214081354.56605-2-wqu@suse.com
State New
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Series
  • Btrfs: relocation: Refactor build_backref_tree() using btrfs_backref_iterator infrastructure
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Commit Message

Qu Wenruo Feb. 14, 2020, 8:13 a.m. UTC
Due to the complex nature of btrfs extent tree, when we want to iterate
all backrefs of one extent, it involves quite a lot of works, like
search the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
backrefs.

Normally this would result pretty complex code, something like:
  btrfs_search_slot()
  /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
  while (1) {	/* Loop for extent tree items */
	while (ptr < end) { /* Loop for inlined items */
		/* REAL WORK HERE */
	}
  next:
  	ret = btrfs_next_item()
	/* Ensure we're still at keyed item for specified bytenr */
  }

The idea of btrfs_backref_iterator is to avoid such complex and hard to
read code structure, but something like the following:

  iterator = btrfs_backref_iterator_alloc();
  ret = btrfs_backref_iterator_start(iterator, bytenr);
  if (ret < 0)
	goto out;
  for (; ; ret = btrfs_backref_iterator_next(iterator)) {
	/* REAL WORK HERE */
  }
  out:
  btrfs_backref_iterator_free(iterator);

This patch is just the skeleton + btrfs_backref_iterator_start() code.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/backref.c | 58 +++++++++++++++++++++++++++++++++++
 fs/btrfs/backref.h | 76 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)

Comments

Johannes Thumshirn Feb. 14, 2020, 8:53 a.m. UTC | #1
On 14/02/2020 09:14, Qu Wenruo wrote:
> Due to the complex nature of btrfs extent tree, when we want to iterate
> all backrefs of one extent, it involves quite a lot of works, like
                                                 Nit: work ^

> search the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
   ^ searching I think but a native English speaker might want to double 
check on that.

[...]

> The idea of btrfs_backref_iterator is to avoid such complex and hard to
> read code structure, but something like the following:
> 
>    iterator = btrfs_backref_iterator_alloc();
>    ret = btrfs_backref_iterator_start(iterator, bytenr);
>    if (ret < 0)
> 	goto out;
>    for (; ; ret = btrfs_backref_iterator_next(iterator)) {
> 	/* REAL WORK HERE */
>    }
>    out:
>    btrfs_backref_iterator_free(iterator);


I personally like for each style macros to wrap these a lot, but seeing 
the loop is only used once in your patchset I'm not sure it's worth 
adding it.
Nikolay Borisov Feb. 14, 2020, 9:19 a.m. UTC | #2
On 14.02.20 г. 10:13 ч., Qu Wenruo wrote:
> Due to the complex nature of btrfs extent tree, when we want to iterate
> all backrefs of one extent, it involves quite a lot of works, like
> search the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
> backrefs.
> 
> Normally this would result pretty complex code, something like:
>   btrfs_search_slot()
>   /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
>   while (1) {	/* Loop for extent tree items */
> 	while (ptr < end) { /* Loop for inlined items */
> 		/* REAL WORK HERE */
> 	}
>   next:
>   	ret = btrfs_next_item()
> 	/* Ensure we're still at keyed item for specified bytenr */
>   }
> 
> The idea of btrfs_backref_iterator is to avoid such complex and hard to
> read code structure, but something like the following:
> 
>   iterator = btrfs_backref_iterator_alloc();
>   ret = btrfs_backref_iterator_start(iterator, bytenr);
>   if (ret < 0)
> 	goto out;
>   for (; ; ret = btrfs_backref_iterator_next(iterator)) {
> 	/* REAL WORK HERE */
>   }
>   out:
>   btrfs_backref_iterator_free(iterator);
> 
> This patch is just the skeleton + btrfs_backref_iterator_start() code.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/backref.c | 58 +++++++++++++++++++++++++++++++++++
>  fs/btrfs/backref.h | 76 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 134 insertions(+)
> 
> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> index e5d85311d5d5..73c4829609c9 100644
> --- a/fs/btrfs/backref.c
> +++ b/fs/btrfs/backref.c
> @@ -2252,3 +2252,61 @@ void free_ipath(struct inode_fs_paths *ipath)
>  	kvfree(ipath->fspath);
>  	kfree(ipath);
>  }
> +
> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
> +				 u64 bytenr)
> +{
> +	struct btrfs_fs_info *fs_info = iterator->fs_info;
> +	struct btrfs_path *path = iterator->path;
> +	struct btrfs_extent_item *ei;
> +	struct btrfs_key key;
> +	int ret;
> +
> +	key.objectid = bytenr;
> +	key.type = BTRFS_METADATA_ITEM_KEY;
> +	key.offset = (u64)-1;
> +
> +	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
> +	if (ret < 0)
> +		return ret;
> +	if (ret == 0) {
> +		ret = -EUCLEAN;
> +		goto release;
> +	}
> +	if (path->slots[0] == 0) {
> +		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
> +		ret = -EUCLEAN;
> +		goto release;
> +	}
> +	path->slots[0]--;
> +
> +	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
> +	if (!(key.type == BTRFS_EXTENT_ITEM_KEY ||
> +	      key.type == BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
> +		ret = -ENOENT;
> +		goto release;
> +	}
> +	memcpy(&iterator->cur_key, &key, sizeof(key));
> +	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
> +	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
> +						   path->slots[0]);
> +	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
> +			    struct btrfs_extent_item);
> +
> +	/* Only support iteration on tree backref yet */
> +	if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
> +		ret = -ENOTTY;
> +		goto release;
> +	}

Isn't this implied bye the fact you are searching for METADATA ITEMS to
begin with ? Considering this shouldn't detecting EXTENT_FLAG_DATA in
the backrefs of a METADATA_EXTENT be considered a corruption?

> +	iterator->cur_ptr = iterator->item_ptr + sizeof(*ei);
> +	return 0;
> +release:
> +	iterator->end_ptr = 0;
> +	iterator->cur_ptr = 0;
> +	iterator->item_ptr = 0;
> +	iterator->cur_key.objectid = 0;
> +	iterator->cur_key.type = 0;
> +	iterator->cur_key.offset = 0;
> +	btrfs_release_path(path);
> +	return ret;
> +}
> diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
> index 777f61dc081e..b53301f2147f 100644
> --- a/fs/btrfs/backref.h
> +++ b/fs/btrfs/backref.h
> @@ -74,4 +74,80 @@ struct prelim_ref {
>  	u64 wanted_disk_byte;
>  };
>  
> +/*
> + * Helper structure to help iterate backrefs of one extent.
> + *
> + * Now it only supports iteration for tree block in commit root.
> + */
> +struct btrfs_backref_iterator {
> +	u64 bytenr;
> +	struct btrfs_path *path;
> +	struct btrfs_fs_info *fs_info;
> +	struct btrfs_key cur_key;
> +	unsigned long item_ptr;
> +	unsigned long cur_ptr;
> +	unsigned long end_ptr;
> +};
> +
> +static inline struct btrfs_backref_iterator *
> +btrfs_backref_iterator_alloc(struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
> +{
> +	struct btrfs_backref_iterator *ret;
> +
> +	ret = kzalloc(sizeof(*ret), gfp_flag);
> +	if (!ret)
> +		return NULL;
> +
> +	ret->path = btrfs_alloc_path();
> +	if (!ret) {
> +		kfree(ret);
> +		return NULL;
> +	}
> +
> +	/* Current backref iterator only supports iteration in commit root */
> +	ret->path->search_commit_root = 1;
> +	ret->path->skip_locking = 1;
> +	ret->path->reada = READA_FORWARD;
> +	ret->fs_info = fs_info;
> +
> +	return ret;
> +}
> +
> +static inline void btrfs_backref_iterator_free(
> +		struct btrfs_backref_iterator *iterator)
> +{
> +	if (!iterator)
> +		return;
> +	btrfs_free_path(iterator->path);
> +	kfree(iterator);
> +}
> +
> +static inline struct extent_buffer *
> +btrfs_backref_get_eb(struct btrfs_backref_iterator *iterator)
> +{
> +	if (!iterator)
> +		return NULL;
> +	return iterator->path->nodes[0];
> +}
> +
> +/*
> + * For metadata with EXTENT_ITEM key (non-skinny) case, the first inline data
> + * is btrfs_tree_block_info, without a btrfs_extent_inline_ref header.
> + *
> + * This helper is here to determine if that's the case.
> + */
> +static inline bool btrfs_backref_has_tree_block_info(
> +		struct btrfs_backref_iterator *iterator)
> +{
> +	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
> +	    iterator->cur_ptr - iterator->item_ptr ==
> +	    sizeof(struct btrfs_extent_item))
> +		return true;
> +	return false;
> +}
> +
> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
> +				 u64 bytenr);
> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);
> +
>  #endif
>
Nikolay Borisov Feb. 14, 2020, 9:24 a.m. UTC | #3
<snip>

> + * This helper is here to determine if that's the case.
> + */
> +static inline bool btrfs_backref_has_tree_block_info(
> +		struct btrfs_backref_iterator *iterator)
> +{
> +	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
> +	    iterator->cur_ptr - iterator->item_ptr ==
> +	    sizeof(struct btrfs_extent_item))
> +		return true;
> +	return false;
> +}
> +
> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
> +				 u64 bytenr);
> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);

You are exposing a function here which is not implemented. Remove this
line and add it in the next patch where you actually introduce
iterator_next.

> +
>  #endif
>
Qu Wenruo Feb. 14, 2020, 9:33 a.m. UTC | #4
On 2020/2/14 下午5:19, Nikolay Borisov wrote:
>
>
> On 14.02.20 г. 10:13 ч., Qu Wenruo wrote:
>> Due to the complex nature of btrfs extent tree, when we want to iterate
>> all backrefs of one extent, it involves quite a lot of works, like
>> search the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
>> backrefs.
>>
>> Normally this would result pretty complex code, something like:
>>   btrfs_search_slot()
>>   /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
>>   while (1) {	/* Loop for extent tree items */
>> 	while (ptr < end) { /* Loop for inlined items */
>> 		/* REAL WORK HERE */
>> 	}
>>   next:
>>   	ret = btrfs_next_item()
>> 	/* Ensure we're still at keyed item for specified bytenr */
>>   }
>>
>> The idea of btrfs_backref_iterator is to avoid such complex and hard to
>> read code structure, but something like the following:
>>
>>   iterator = btrfs_backref_iterator_alloc();
>>   ret = btrfs_backref_iterator_start(iterator, bytenr);
>>   if (ret < 0)
>> 	goto out;
>>   for (; ; ret = btrfs_backref_iterator_next(iterator)) {
>> 	/* REAL WORK HERE */
>>   }
>>   out:
>>   btrfs_backref_iterator_free(iterator);
>>
>> This patch is just the skeleton + btrfs_backref_iterator_start() code.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/backref.c | 58 +++++++++++++++++++++++++++++++++++
>>  fs/btrfs/backref.h | 76 ++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 134 insertions(+)
>>
>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>> index e5d85311d5d5..73c4829609c9 100644
>> --- a/fs/btrfs/backref.c
>> +++ b/fs/btrfs/backref.c
>> @@ -2252,3 +2252,61 @@ void free_ipath(struct inode_fs_paths *ipath)
>>  	kvfree(ipath->fspath);
>>  	kfree(ipath);
>>  }
>> +
>> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>> +				 u64 bytenr)
>> +{
>> +	struct btrfs_fs_info *fs_info = iterator->fs_info;
>> +	struct btrfs_path *path = iterator->path;
>> +	struct btrfs_extent_item *ei;
>> +	struct btrfs_key key;
>> +	int ret;
>> +
>> +	key.objectid = bytenr;
>> +	key.type = BTRFS_METADATA_ITEM_KEY;
>> +	key.offset = (u64)-1;
>> +
>> +	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
>> +	if (ret < 0)
>> +		return ret;
>> +	if (ret == 0) {
>> +		ret = -EUCLEAN;
>> +		goto release;
>> +	}
>> +	if (path->slots[0] == 0) {
>> +		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
>> +		ret = -EUCLEAN;
>> +		goto release;
>> +	}
>> +	path->slots[0]--;
>> +
>> +	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
>> +	if (!(key.type == BTRFS_EXTENT_ITEM_KEY ||
>> +	      key.type == BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
>> +		ret = -ENOENT;
>> +		goto release;
>> +	}
>> +	memcpy(&iterator->cur_key, &key, sizeof(key));
>> +	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
>> +	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
>> +						   path->slots[0]);
>> +	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
>> +			    struct btrfs_extent_item);
>> +
>> +	/* Only support iteration on tree backref yet */
>> +	if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
>> +		ret = -ENOTTY;
>> +		goto release;
>> +	}
>
> Isn't this implied bye the fact you are searching for METADATA ITEMS to
> begin with ? Considering this shouldn't detecting EXTENT_FLAG_DATA in
> the backrefs of a METADATA_EXTENT be considered a corruption?

For non skinny-metadata fs, we can hit with EXTENT_ITEM.
So it's still possible to hit a corruption undetected by tree-checker.

But you're right, we shouldn't really hit a data extent here, as
previous loops have excluded all data extents.

But I'm just the guy who want to be extra cautious.

Thanks,
Qu

>
>> +	iterator->cur_ptr = iterator->item_ptr + sizeof(*ei);
>> +	return 0;
>> +release:
>> +	iterator->end_ptr = 0;
>> +	iterator->cur_ptr = 0;
>> +	iterator->item_ptr = 0;
>> +	iterator->cur_key.objectid = 0;
>> +	iterator->cur_key.type = 0;
>> +	iterator->cur_key.offset = 0;
>> +	btrfs_release_path(path);
>> +	return ret;
>> +}
>> diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
>> index 777f61dc081e..b53301f2147f 100644
>> --- a/fs/btrfs/backref.h
>> +++ b/fs/btrfs/backref.h
>> @@ -74,4 +74,80 @@ struct prelim_ref {
>>  	u64 wanted_disk_byte;
>>  };
>>
>> +/*
>> + * Helper structure to help iterate backrefs of one extent.
>> + *
>> + * Now it only supports iteration for tree block in commit root.
>> + */
>> +struct btrfs_backref_iterator {
>> +	u64 bytenr;
>> +	struct btrfs_path *path;
>> +	struct btrfs_fs_info *fs_info;
>> +	struct btrfs_key cur_key;
>> +	unsigned long item_ptr;
>> +	unsigned long cur_ptr;
>> +	unsigned long end_ptr;
>> +};
>> +
>> +static inline struct btrfs_backref_iterator *
>> +btrfs_backref_iterator_alloc(struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
>> +{
>> +	struct btrfs_backref_iterator *ret;
>> +
>> +	ret = kzalloc(sizeof(*ret), gfp_flag);
>> +	if (!ret)
>> +		return NULL;
>> +
>> +	ret->path = btrfs_alloc_path();
>> +	if (!ret) {
>> +		kfree(ret);
>> +		return NULL;
>> +	}
>> +
>> +	/* Current backref iterator only supports iteration in commit root */
>> +	ret->path->search_commit_root = 1;
>> +	ret->path->skip_locking = 1;
>> +	ret->path->reada = READA_FORWARD;
>> +	ret->fs_info = fs_info;
>> +
>> +	return ret;
>> +}
>> +
>> +static inline void btrfs_backref_iterator_free(
>> +		struct btrfs_backref_iterator *iterator)
>> +{
>> +	if (!iterator)
>> +		return;
>> +	btrfs_free_path(iterator->path);
>> +	kfree(iterator);
>> +}
>> +
>> +static inline struct extent_buffer *
>> +btrfs_backref_get_eb(struct btrfs_backref_iterator *iterator)
>> +{
>> +	if (!iterator)
>> +		return NULL;
>> +	return iterator->path->nodes[0];
>> +}
>> +
>> +/*
>> + * For metadata with EXTENT_ITEM key (non-skinny) case, the first inline data
>> + * is btrfs_tree_block_info, without a btrfs_extent_inline_ref header.
>> + *
>> + * This helper is here to determine if that's the case.
>> + */
>> +static inline bool btrfs_backref_has_tree_block_info(
>> +		struct btrfs_backref_iterator *iterator)
>> +{
>> +	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
>> +	    iterator->cur_ptr - iterator->item_ptr ==
>> +	    sizeof(struct btrfs_extent_item))
>> +		return true;
>> +	return false;
>> +}
>> +
>> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>> +				 u64 bytenr);
>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);
>> +
>>  #endif
>>
Qu Wenruo Feb. 14, 2020, 9:34 a.m. UTC | #5
On 2020/2/14 下午5:24, Nikolay Borisov wrote:
> <snip>
>
>> + * This helper is here to determine if that's the case.
>> + */
>> +static inline bool btrfs_backref_has_tree_block_info(
>> +		struct btrfs_backref_iterator *iterator)
>> +{
>> +	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
>> +	    iterator->cur_ptr - iterator->item_ptr ==
>> +	    sizeof(struct btrfs_extent_item))
>> +		return true;
>> +	return false;
>> +}
>> +
>> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>> +				 u64 bytenr);
>> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);
>
> You are exposing a function here which is not implemented. Remove this
> line and add it in the next patch where you actually introduce
> iterator_next.

OK, I'll take this as a generic principle for later patches.

Thanks,
Qu
>
>> +
>>  #endif
>>
Nikolay Borisov Feb. 14, 2020, 9:36 a.m. UTC | #6
On 14.02.20 г. 11:33 ч., Qu Wenruo wrote:
> 
> 
> On 2020/2/14 下午5:19, Nikolay Borisov wrote:
>>
>>
>> On 14.02.20 г. 10:13 ч., Qu Wenruo wrote:
>>> Due to the complex nature of btrfs extent tree, when we want to iterate
>>> all backrefs of one extent, it involves quite a lot of works, like
>>> search the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
>>> backrefs.
>>>
>>> Normally this would result pretty complex code, something like:
>>>   btrfs_search_slot()
>>>   /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
>>>   while (1) {	/* Loop for extent tree items */
>>> 	while (ptr < end) { /* Loop for inlined items */
>>> 		/* REAL WORK HERE */
>>> 	}
>>>   next:
>>>   	ret = btrfs_next_item()
>>> 	/* Ensure we're still at keyed item for specified bytenr */
>>>   }
>>>
>>> The idea of btrfs_backref_iterator is to avoid such complex and hard to
>>> read code structure, but something like the following:
>>>
>>>   iterator = btrfs_backref_iterator_alloc();
>>>   ret = btrfs_backref_iterator_start(iterator, bytenr);
>>>   if (ret < 0)
>>> 	goto out;
>>>   for (; ; ret = btrfs_backref_iterator_next(iterator)) {
>>> 	/* REAL WORK HERE */
>>>   }
>>>   out:
>>>   btrfs_backref_iterator_free(iterator);
>>>
>>> This patch is just the skeleton + btrfs_backref_iterator_start() code.
>>>
>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>> ---
>>>  fs/btrfs/backref.c | 58 +++++++++++++++++++++++++++++++++++
>>>  fs/btrfs/backref.h | 76 ++++++++++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 134 insertions(+)
>>>
>>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>>> index e5d85311d5d5..73c4829609c9 100644
>>> --- a/fs/btrfs/backref.c
>>> +++ b/fs/btrfs/backref.c
>>> @@ -2252,3 +2252,61 @@ void free_ipath(struct inode_fs_paths *ipath)
>>>  	kvfree(ipath->fspath);
>>>  	kfree(ipath);
>>>  }
>>> +
>>> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
>>> +				 u64 bytenr)
>>> +{
>>> +	struct btrfs_fs_info *fs_info = iterator->fs_info;
>>> +	struct btrfs_path *path = iterator->path;
>>> +	struct btrfs_extent_item *ei;
>>> +	struct btrfs_key key;
>>> +	int ret;
>>> +
>>> +	key.objectid = bytenr;
>>> +	key.type = BTRFS_METADATA_ITEM_KEY;
>>> +	key.offset = (u64)-1;
>>> +
>>> +	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
>>> +	if (ret < 0)
>>> +		return ret;
>>> +	if (ret == 0) {
>>> +		ret = -EUCLEAN;
>>> +		goto release;
>>> +	}
>>> +	if (path->slots[0] == 0) {
>>> +		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
>>> +		ret = -EUCLEAN;
>>> +		goto release;
>>> +	}
>>> +	path->slots[0]--;
>>> +
>>> +	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
>>> +	if (!(key.type == BTRFS_EXTENT_ITEM_KEY ||
>>> +	      key.type == BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
>>> +		ret = -ENOENT;
>>> +		goto release;
>>> +	}
>>> +	memcpy(&iterator->cur_key, &key, sizeof(key));
>>> +	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
>>> +	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
>>> +						   path->slots[0]);
>>> +	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
>>> +			    struct btrfs_extent_item);
>>> +
>>> +	/* Only support iteration on tree backref yet */
>>> +	if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
>>> +		ret = -ENOTTY;
>>> +		goto release;
>>> +	}
>>
>> Isn't this implied bye the fact you are searching for METADATA ITEMS to
>> begin with ? Considering this shouldn't detecting EXTENT_FLAG_DATA in
>> the backrefs of a METADATA_EXTENT be considered a corruption?
> 
> For non skinny-metadata fs, we can hit with EXTENT_ITEM.
> So it's still possible to hit a corruption undetected by tree-checker.
> 
> But you're right, we shouldn't really hit a data extent here, as
> previous loops have excluded all data extents.

Then put a comment saying this is done as an extra precaution.

<split>

Patch
diff mbox series

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index e5d85311d5d5..73c4829609c9 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -2252,3 +2252,61 @@  void free_ipath(struct inode_fs_paths *ipath)
 	kvfree(ipath->fspath);
 	kfree(ipath);
 }
+
+int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
+				 u64 bytenr)
+{
+	struct btrfs_fs_info *fs_info = iterator->fs_info;
+	struct btrfs_path *path = iterator->path;
+	struct btrfs_extent_item *ei;
+	struct btrfs_key key;
+	int ret;
+
+	key.objectid = bytenr;
+	key.type = BTRFS_METADATA_ITEM_KEY;
+	key.offset = (u64)-1;
+
+	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
+	if (ret < 0)
+		return ret;
+	if (ret == 0) {
+		ret = -EUCLEAN;
+		goto release;
+	}
+	if (path->slots[0] == 0) {
+		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
+		ret = -EUCLEAN;
+		goto release;
+	}
+	path->slots[0]--;
+
+	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+	if (!(key.type == BTRFS_EXTENT_ITEM_KEY ||
+	      key.type == BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
+		ret = -ENOENT;
+		goto release;
+	}
+	memcpy(&iterator->cur_key, &key, sizeof(key));
+	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
+	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
+						   path->slots[0]);
+	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
+			    struct btrfs_extent_item);
+
+	/* Only support iteration on tree backref yet */
+	if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
+		ret = -ENOTTY;
+		goto release;
+	}
+	iterator->cur_ptr = iterator->item_ptr + sizeof(*ei);
+	return 0;
+release:
+	iterator->end_ptr = 0;
+	iterator->cur_ptr = 0;
+	iterator->item_ptr = 0;
+	iterator->cur_key.objectid = 0;
+	iterator->cur_key.type = 0;
+	iterator->cur_key.offset = 0;
+	btrfs_release_path(path);
+	return ret;
+}
diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
index 777f61dc081e..b53301f2147f 100644
--- a/fs/btrfs/backref.h
+++ b/fs/btrfs/backref.h
@@ -74,4 +74,80 @@  struct prelim_ref {
 	u64 wanted_disk_byte;
 };
 
+/*
+ * Helper structure to help iterate backrefs of one extent.
+ *
+ * Now it only supports iteration for tree block in commit root.
+ */
+struct btrfs_backref_iterator {
+	u64 bytenr;
+	struct btrfs_path *path;
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_key cur_key;
+	unsigned long item_ptr;
+	unsigned long cur_ptr;
+	unsigned long end_ptr;
+};
+
+static inline struct btrfs_backref_iterator *
+btrfs_backref_iterator_alloc(struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
+{
+	struct btrfs_backref_iterator *ret;
+
+	ret = kzalloc(sizeof(*ret), gfp_flag);
+	if (!ret)
+		return NULL;
+
+	ret->path = btrfs_alloc_path();
+	if (!ret) {
+		kfree(ret);
+		return NULL;
+	}
+
+	/* Current backref iterator only supports iteration in commit root */
+	ret->path->search_commit_root = 1;
+	ret->path->skip_locking = 1;
+	ret->path->reada = READA_FORWARD;
+	ret->fs_info = fs_info;
+
+	return ret;
+}
+
+static inline void btrfs_backref_iterator_free(
+		struct btrfs_backref_iterator *iterator)
+{
+	if (!iterator)
+		return;
+	btrfs_free_path(iterator->path);
+	kfree(iterator);
+}
+
+static inline struct extent_buffer *
+btrfs_backref_get_eb(struct btrfs_backref_iterator *iterator)
+{
+	if (!iterator)
+		return NULL;
+	return iterator->path->nodes[0];
+}
+
+/*
+ * For metadata with EXTENT_ITEM key (non-skinny) case, the first inline data
+ * is btrfs_tree_block_info, without a btrfs_extent_inline_ref header.
+ *
+ * This helper is here to determine if that's the case.
+ */
+static inline bool btrfs_backref_has_tree_block_info(
+		struct btrfs_backref_iterator *iterator)
+{
+	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
+	    iterator->cur_ptr - iterator->item_ptr ==
+	    sizeof(struct btrfs_extent_item))
+		return true;
+	return false;
+}
+
+int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
+				 u64 bytenr);
+int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);
+
 #endif