On 2018年05月22日 15:49, Nikolay Borisov wrote:
>
>
> On 22.05.2018 10:45, Qu Wenruo wrote:
>>
>>
>> On 2018年05月22日 15:37, Nikolay Borisov wrote:
>>>
>>>
>>> On 22.05.2018 10:29, Qu Wenruo wrote:
>>>> Introduce a small helper, btrfs_add_unused_bgs(), to accquire needed
>>>
>>> This function name sounds a bit awkard, mainly because you use the
>>> plural form. How about btrfs_mark_bg_unused() ? The name seems more
>>> unambiguous.
>>
>> Sounds much better.
>>
>>>
>>>> locks and add a block group to unused_bgs list.
>>>>
>>>> No functional modification, and only 3 callers are involved.
>>>>
>>>> Signed-off-by: Qu Wenruo <w...@suse.com>
>>>> ---
>>>> This patch should provide the basis for later block group auto-removal
>>>> to get more info (mostly transid) to determine should one block group
>>>> being removed in current trans.
>>>> ---
>>>> fs/btrfs/ctree.h | 1 +
>>>> fs/btrfs/extent-tree.c | 35 ++++++++++++++++-------------------
>>>> fs/btrfs/scrub.c | 9 +--------
>>>> 3 files changed, 18 insertions(+), 27 deletions(-)
>>>>
>>>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>>>> index bbb358143ded..701a52034ec6 100644
>>>> --- a/fs/btrfs/ctree.h
>>>> +++ b/fs/btrfs/ctree.h
>>>> @@ -2827,6 +2827,7 @@ void check_system_chunk(struct btrfs_trans_handle
>>>> *trans,
>>>> struct btrfs_fs_info *fs_info, const u64 type);
>>>> u64 add_new_free_space(struct btrfs_block_group_cache *block_group,
>>>> u64 start, u64 end);
>>>> +void btrfs_add_unused_bgs(struct btrfs_block_group_cache *bg);
>>>>
>>>> /* ctree.c */
>>>> int btrfs_bin_search(struct extent_buffer *eb, const struct btrfs_key
>>>> *key,
>>>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>>>> index ccf2690f7ca1..484c9d11e5b6 100644
>>>> --- a/fs/btrfs/extent-tree.c
>>>> +++ b/fs/btrfs/extent-tree.c
>>>> @@ -6312,16 +6312,8 @@ static int update_block_group(struct
>>>> btrfs_trans_handle *trans,
>>>> * dirty list to avoid races between cleaner kthread and space
>>>> * cache writeout.
>>>> */
>>>> - if (!alloc && old_val == 0) {
>>>> - spin_lock(&info->unused_bgs_lock);
>>>> - if (list_empty(&cache->bg_list)) {
>>>> - btrfs_get_block_group(cache);
>>>> - trace_btrfs_add_unused_block_group(cache);
>>>> - list_add_tail(&cache->bg_list,
>>>> - &info->unused_bgs);
>>>> - }
>>>> - spin_unlock(&info->unused_bgs_lock);
>>>> - }
>>>> + if (!alloc && old_val == 0)
>>>> + btrfs_add_unused_bgs(cache);
>>>>
>>>> btrfs_put_block_group(cache);
>>>> total -= num_bytes;
>>>> @@ -10144,15 +10136,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info
>>>> *info)
>>>> if (btrfs_chunk_readonly(info, cache->key.objectid)) {
>>>> inc_block_group_ro(cache, 1);
>>>> } else if (btrfs_block_group_used(&cache->item) == 0) {
>>>> - spin_lock(&info->unused_bgs_lock);
>>>> - /* Should always be true but just in case. */
>>>> - if (list_empty(&cache->bg_list)) {
>>>> - btrfs_get_block_group(cache);
>>>> - trace_btrfs_add_unused_block_group(cache);
>>>> - list_add_tail(&cache->bg_list,
>>>> - &info->unused_bgs);
>>>> - }
>>>> - spin_unlock(&info->unused_bgs_lock);
>>>> + btrfs_add_unused_bgs(cache);
>>>> }
>>>> }
>>>>
>>>> @@ -11071,3 +11055,16 @@ void btrfs_wait_for_snapshot_creation(struct
>>>> btrfs_root *root)
>>>> !atomic_read(&root->will_be_snapshotted));
>>>> }
>>>> }
>>>> +
>>>> +void btrfs_add_unused_bgs(struct btrfs_block_group_cache *bg)
>>>> +{
>>>> + struct btrfs_fs_info *fs_info = bg->fs_info;
>>>> +
>>>> + spin_lock(&fs_info->unused_bgs_lock);
>>>> + if (list_empty(&bg->bg_list)) {
>>>
>>> Given the comment in btrfs_read_block_groups:
>>>
>>> /* Should always be true but just in case. */
>>>
>>> How about you make it ASSERT(list_empty(&bg->bg_list));
>>>
>>> /* code to add the bg */
>>>
>>> So right now either :
>>>
>>> a) The comment is bogus and it is indeed required to check if this bg
>>> has already been marked unused.
>>>
>>> or
>>>
>>> b) The comment is correct and it's in fact a bug to try and mark a bg as
>>> unused twice.
>>
>> Not exactly.
>>
>> 1) bg_list is kind of abused.
>> Not only fs_info->unused_bgs, but also transaction->deleted_bgs, and
>> even transaction->new_bgs could use bg_cache->bg_list.
>> So it's not only used to detect unused bgs.
>> And it's possible some bg get moved to deleted_bgs list.
>
> I haven't looked at the code but if this is indeed the case then doesn't
> it make sense to try and fix this abuse, otherwise don't we risk
> processing a bg in the wrong context? In other words, shouldn't bgs have
> 1 list member for every list they could be part of?I guess a single list
> member would have made sense IFF there was 1 central place where this
> list manipulation was performed, which currently there isn't, yes?
Makes sense for btrfs_read_block_groups() caller.
I'll add that assert at btrfs_read_block_groups().
Thanks,
Qu
>
>>
>> 2) That is comment only works for caller in btrfs_read_block_groups().
>> As at that timing, there is no race at all since we're still mounting
>> the fs.
>> But may not work for other callers.
>>
>> Thus I just kept the code while removed the comment, since in the
>> extracted function, it may no longer be the case.
>> (And my focus is later auto-removal generation check, so I just left
>> code as is)
>>
>> Thanks,
>> Qu
>>
>>>
>>>> + btrfs_get_block_group(bg);
>>>> + trace_btrfs_add_unused_block_group(bg);
>>>> + list_add_tail(&bg->bg_list, &fs_info->unused_bgs);
>>>> + }
>>>> + spin_unlock(&fs_info->unused_bgs_lock);
>>>> +}
>>>> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
>>>> index a59005862010..1044ab2fc71c 100644
>>>> --- a/fs/btrfs/scrub.c
>>>> +++ b/fs/btrfs/scrub.c
>>>> @@ -3981,14 +3981,7 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
>>>> if (!cache->removed && !cache->ro && cache->reserved == 0 &&
>>>> btrfs_block_group_used(&cache->item) == 0) {
>>>> spin_unlock(&cache->lock);
>>>> - spin_lock(&fs_info->unused_bgs_lock);
>>>> - if (list_empty(&cache->bg_list)) {
>>>> - btrfs_get_block_group(cache);
>>>> - trace_btrfs_add_unused_block_group(cache);
>>>> - list_add_tail(&cache->bg_list,
>>>> - &fs_info->unused_bgs);
>>>> - }
>>>> - spin_unlock(&fs_info->unused_bgs_lock);
>>>> + btrfs_add_unused_bgs(cache);
>>>> } else {
>>>> spin_unlock(&cache->lock);
>>>> }
>>>>
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