Re: [PATCH v2 06/17] btrfs-progs: lowmem check: introduce force_cow_in_new_chunk()

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

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On 2017年12月20日 16:37, Qu Wenruo wrote:
> 
> 
> On 2017年12月20日 16:21, Su Yue wrote:
>>
>>
>> On 12/20/2017 01:41 PM, Qu Wenruo wrote:
>>>
>>>
>>> On 2017年12月20日 12:57, Su Yue wrote:
>>>> Introduce create_chunk_and_block_block_group() to allocate new chunk
>>>> and corresponding block group.
>>>>
>>>> The new function force_cow_in_new_chunk() first allocates new chunk
>>>> and records its start.
>>>> Then it modifies all metadata block groups cached and full.
>>>> Finally it marks the new block group uncached and unfree.
>>>> In the next CoW, extents states will be updated automatically by
>>>> cache_block_group().
>>>>
>>>> Suggested-by: Qu Wenruo <wqu@suse.com>
>>>> Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
>>>> ---
>>>>   cmds-check.c | 80
>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>   1 file changed, 80 insertions(+)
>>>>
>>>> diff --git a/cmds-check.c b/cmds-check.c
>>>> index d98d4bda7357..311c8a9f45e8 100644
>>>> --- a/cmds-check.c
>>>> +++ b/cmds-check.c
>>>> @@ -10911,6 +10911,86 @@ out:
>>>>       return ret;
>>>>   }
>>>>   +static int create_chunk_and_block_group(struct btrfs_fs_info
>>>> *fs_info,
>>>> +                    u64 flags, u64 *start, u64 *nbytes)
>>>> +{
>>>> +    struct btrfs_trans_handle *trans;
>>>> +    struct btrfs_root *root = fs_info->extent_root;
>>>> +    int ret;
>>>> +
>>>> +    if ((flags & BTRFS_BLOCK_GROUP_TYPE_MASK) == 0)
>>>> +        return -EINVAL;
>>>> +
>>>> +    trans = btrfs_start_transaction(root, 1);
>>>> +    if (IS_ERR(trans)) {
>>>> +        ret = PTR_ERR(trans);
>>>> +        error("error starting transaction %s", strerror(-ret));
>>>> +        return ret;
>>>> +    }
>>>> +    ret = btrfs_alloc_chunk(trans, fs_info, start, nbytes, flags);
>>>> +    if (ret) {
>>>> +        error("fail to allocate new chunk %s", strerror(-ret));
>>>> +        goto out;
>>>> +    }
>>>> +    ret = btrfs_make_block_group(trans, fs_info, 0, flags,
>>>> +                 BTRFS_FIRST_CHUNK_TREE_OBJECTID, *start, *nbytes);
>>>> +    if (ret) {
>>>> +        error("fail to make block group for chunk %llu %llu %s",
>>>> +              *start, *nbytes, strerror(-ret));
>>>> +        goto out;
>>>> +    }
>>>> +out:
>>>> +    btrfs_commit_transaction(trans, root);
>>>> +    return ret;
>>>> +}
>>>> +
>>>> +static int force_cow_in_new_chunk(struct btrfs_fs_info *fs_info)
>>>> +{
>>>> +    struct btrfs_block_group_cache *bg;
>>>> +    u64 start;
>>>> +    u64 nbytes;
>>>> +    u64 alloc_profile;
>>>> +    u64 flags;
>>>> +    int ret;
>>>> +
>>>> +    alloc_profile = (fs_info->avail_metadata_alloc_bits &
>>>> +             fs_info->metadata_alloc_profile);
>>>> +    flags = BTRFS_BLOCK_GROUP_METADATA | alloc_profile;
>>>> +    if (btrfs_fs_incompat(fs_info, MIXED_GROUPS))
>>>> +        flags |= BTRFS_BLOCK_GROUP_DATA;
>>>> +
>>>> +    ret = create_chunk_and_block_group(fs_info, flags, &start,
>>>> &nbytes);
>>>
>>> Why bother allocating a new chunk by yourself?
>>>
>>> Just mark all block groups full and that's all.
>>>
>>> Any later tree block allocation like btrfs_search_slot() with @cow = 1
>>> will trigger chunk allocation automatically.
>>>
>>
>> Tried to let it happen but BUG_ON with -ENOSPC.
> 
> Then fix it.
> 
> It's not a normal behavior in this case.
> 
> Thanks,
> Qu

And I think the fix to allow btrfs_reserve_extent() to allocate new
chunk will solve a lot of strange BUG_ON().

it just occurred to me that, a lot of use cases relies on the assumption
that btrfs_reserve_extent() will try to allocate new chunks.

Especially for case like convert, certain btrfs check --repair, some
rescue tools.

So this would be a quite nice start point for such fix.

Thanks,
Qu

> 
>> Do you mean do_chunk_alloc() in btrfs_reserve_extent() which is called
>> while doing CoW?> In progs, allocation of new chunk during CoW depends @root->ref_cows.
>> However, @root->ref_cows should be set only if @root is root of a fs
>> trees.
>>
>> Thanks,
>> Su
>>
>>> Thanks,
>>> Qu
>>>
>>>> +    if (!ret)
>>>> +        printf("Create new chunk %llu %llu\n", start, nbytes);
>>>> +    else
>>>> +        goto err;
>>>> +
>>>> +    flags = BTRFS_BLOCK_GROUP_METADATA;
>>>> +    /* Mark all metadata block groups cached and full in free space*/
>>>> +    ret = modify_block_groups_cache(fs_info, flags, 1);
>>>> +    if (ret)
>>>> +        goto clear_bg_cache;
>>>> +
>>>> +    bg = btrfs_lookup_block_group(fs_info, start);
>>>> +    if (!bg) {
>>>> +        ret = -ENOENT;
>>>> +        error("fail to look up block group %llu %llu", start, nbytes);
>>>> +        goto clear_bg_cache;
>>>> +    }
>>>> +
>>>> +    /* Clear block group cache just allocated */
>>>> +    ret = modify_block_group_cache(fs_info, bg, 0);
>>>> +    if (ret)
>>>> +        goto clear_bg_cache;
>>>> +
>>>> +    return 0;
>>>> +
>>>> +clear_bg_cache:
>>>> +    modify_block_groups_cache(fs_info, flags, 0);
>>>> +err:
>>>> +    return ret;
>>>> +}
>>>> +
>>>>   static int check_extent_refs(struct btrfs_root *root,
>>>>                    struct cache_tree *extent_cache)
>>>>   {
>>>>
>>>
>>
>>
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