Re: [PATCH v2] Btrfs: fix race of using total_bytes_pinned

[Miao Xie <miaox@cn.fujitsu.com>]

On Mon, 30 Jun 2014 18:46:07 +0800, Liu Bo wrote:
> This percpu counter @total_bytes_pinned is introduced to skip unnecessary
> operations of 'commit transaction', it accounts for those space we may free
> but are stuck in delayed refs.
> 
> And we zero out @space_info->total_bytes_pinned every transaction period so
> we have a better idea of how much space we'll actually free up by committing
> this transaction.  However, we do the 'zero out' part a little earlier, before
> we actually unpin space, so we end up returning ENOSPC when we actually have
> free space that's just unpinned from committing transaction.
> 
> xfstests/generic/074 complained then.
> 
> This fixes it by zeroing out the percpu pinned number after 'unpin' and when
> finding space for writing, if it finds that pinned bytes is less than needed,
> we first try again to check if we have space now, as someone may have
> committed transaction, yes means we're good, while no means we really have
> run out of space.
> 
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> ---
> v2: add missing brakets of if statement.
> 
>  fs/btrfs/extent-tree.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 99c2539..57793da 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -3685,7 +3685,7 @@ int btrfs_check_data_free_space(struct inode *inode, u64 bytes)
>  	struct btrfs_root *root = BTRFS_I(inode)->root;
>  	struct btrfs_fs_info *fs_info = root->fs_info;
>  	u64 used;
> -	int ret = 0, committed = 0, alloc_chunk = 1;
> +	int ret = 0, committed = 0, alloc_chunk = 1, check_pinned = 0;
>  
>  	/* make sure bytes are sectorsize aligned */
>  	bytes = ALIGN(bytes, root->sectorsize);
> @@ -3756,11 +3756,18 @@ alloc:
>  		 * allocation don't bother committing the transaction.
>  		 */
>  		if (percpu_counter_compare(&data_sinfo->total_bytes_pinned,
> -					   bytes) < 0)
> -			committed = 1;
> +					   bytes) < 0) {
> +			if (check_pinned) {
> +				committed = 1;	/* really run out of space. */
> +			} else {
> +				check_pinned = 1;
> +				spin_unlock(&data_sinfo->lock);
> +				goto again;
> +			}
> +		}
>  		spin_unlock(&data_sinfo->lock);
>  
> -		/* commit the current transaction and try again */
> +	/* commit the current transaction and try again */
>  commit_trans:
>  		if (!committed &&
>  		    !atomic_read(&root->fs_info->open_ioctl_trans)) {
> @@ -5678,7 +5685,6 @@ void btrfs_prepare_extent_commit(struct btrfs_trans_handle *trans,
>  	struct btrfs_caching_control *next;
>  	struct btrfs_caching_control *caching_ctl;
>  	struct btrfs_block_group_cache *cache;
> -	struct btrfs_space_info *space_info;
>  
>  	down_write(&fs_info->commit_root_sem);
>  
> @@ -5701,9 +5707,6 @@ void btrfs_prepare_extent_commit(struct btrfs_trans_handle *trans,
>  
>  	up_write(&fs_info->commit_root_sem);
>  
> -	list_for_each_entry_rcu(space_info, &fs_info->space_info, list)
> -		percpu_counter_set(&space_info->total_bytes_pinned, 0);
> -
>  	update_global_block_rsv(fs_info);
>  }
>  
> @@ -5771,6 +5774,7 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans,
>  {
>  	struct btrfs_fs_info *fs_info = root->fs_info;
>  	struct extent_io_tree *unpin;
> +	struct btrfs_space_info *space_info;
>  	u64 start;
>  	u64 end;
>  	int ret;
> @@ -5798,6 +5802,8 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans,
>  		cond_resched();
>  	}
>  
> +	list_for_each_entry_rcu(space_info, &fs_info->space_info, list)
> +		percpu_counter_set(&space_info->total_bytes_pinned, 0);

This function is called after the transaction is unblocked, that is the new transaction can
be started and some new pinned extents have been accounted into this counter, so we can not
set to 0 directly.

There are several way to avoid the above problem, one is account the pinned space we free
in btrfs_finish_extent_commit then subtract from the counter. The other is introduce a new counter,
and we use those two counter alternately just like freed_extents variant in btrfs_fs_info structure.

Thanks
Miao

>  	return 0;
>  }
>  
>