Re: [PATCH v2 01/10] btrfs: backref: Introduce the skeleton of btrfs_backref_iter

Qu Wenruo
2020-03-04T00:50:40Z

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On 2020/3/4 上午1:19, David Sterba wrote:
> On Mon, Mar 02, 2020 at 05:45:44PM +0800, 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 work, like
>> searching 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_iter is to avoid such complex and hard to
>> read code structure, but something like the following:
>>
>> iter = btrfs_backref_iter_alloc();
>> ret = btrfs_backref_iter_start(iter, bytenr);
>> if (ret < 0)
>> goto out;
>> for (; ; ret = btrfs_backref_iter_next(iter)) {
>> /* REAL WORK HERE */
>> }
>> out:
>> btrfs_backref_iter_free(iter);
>>
>> This patch is just the skeleton + btrfs_backref_iter_start() code.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>> ---
>> fs/btrfs/backref.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++
>> fs/btrfs/backref.h | 60 ++++++++++++++++++++++++++++++++
>> 2 files changed, 147 insertions(+)
>>
>> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>> index 327e4480957b..444cd5d31d87 100644
>> --- a/fs/btrfs/backref.c
>> +++ b/fs/btrfs/backref.c
>> @@ -2299,3 +2299,90 @@ void free_ipath(struct inode_fs_paths *ipath)
>> kvfree(ipath->fspath);
>> kfree(ipath);
>> }
>> +
>> +int btrfs_backref_iter_start(struct btrfs_backref_iter *iter, u64 bytenr)
>> +{
>> + struct btrfs_fs_info *fs_info = iter->fs_info;
>> + struct btrfs_path *path = iter->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;
>> + iter->bytenr = bytenr;
>> +
>> + 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(&iter->cur_key, &key, sizeof(key));
>> + iter->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
>> + path->slots[0]);
>> + iter->end_ptr = iter->item_ptr + btrfs_item_size_nr(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.
>> + *
>> + * This is extra precaustion for non skinny-metadata, where
>> + * EXTENT_ITEM is also used for tree blocks, that we can only use
>> + * extent flags to determine if it's a tree block.
>> + */
>> + if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
>> + ret = -ENOTTY;
>
> What's the reason for ENOTTY?

Because it's not supported yet.

>
>> + goto release;
>> + }
>> + iter->cur_ptr = iter->item_ptr + sizeof(*ei);
>> +
>> + /* If there is no inline backref, go search for keyed backref */
>> + if (iter->cur_ptr >= iter->end_ptr) {
>> + ret = btrfs_next_item(fs_info->extent_root, path);
>> +
>> + /* No inline nor keyed ref */
>> + if (ret > 0) {
>> + ret = -ENOENT;
>> + goto release;
>> + }
>> + if (ret < 0)
>> + goto release;
>> +
>> + btrfs_item_key_to_cpu(path->nodes[0], &iter->cur_key,
>> + path->slots[0]);
>> + if (iter->cur_key.objectid != bytenr ||
>> + (iter->cur_key.type != BTRFS_SHARED_BLOCK_REF_KEY &&
>> + iter->cur_key.type != BTRFS_TREE_BLOCK_REF_KEY)) {
>> + ret = -ENOENT;
>> + goto release;
>> + }
>> + iter->cur_ptr = btrfs_item_ptr_offset(path->nodes[0],
>> + path->slots[0]);
>> + iter->item_ptr = iter->cur_ptr;
>> + iter->end_ptr = iter->item_ptr + btrfs_item_size_nr(
>> + path->nodes[0], path->slots[0]);
>> + }
>> +
>> + return 0;
>> +release:
>> + btrfs_backref_iter_release(iter);
>> + return ret;
>> +}
>> diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
>> index 777f61dc081e..8b1ec11d4b28 100644
>> --- a/fs/btrfs/backref.h
>> +++ b/fs/btrfs/backref.h
>> @@ -74,4 +74,64 @@ 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_iter {
>> + 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_iter *
>> +btrfs_backref_iter_alloc(struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
>
> This does not need to be static inline, and please keep the type and
> function name on the same line.

OK, that makes sense.

Thanks,
Qu
>
>> +{
>> + struct btrfs_backref_iter *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;
>> +}

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