Re: [PATCH v2 1/3] btrfs: backref: Introduce the skeleton of btrfs_backref_iterator

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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
>>