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 <wqu@xxxxxxxx>
>> ---
>> 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.
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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