Re: [PATCH v2 5/7] btrfs-progs: lowmem: continue to check item in last slot while checking inodes

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

On 2018/9/13 上午4:49, damenly.su@gmail.com wrote:
> From: Su Yue <suy.fnst@cn.fujitsu.com>
> 
> After call of check_inode_item(), path may point to the last unchecked
> slot of the leaf. The outer walk_up_tree() always treats the position
> as checked item then skips to next item.

So check_inode_item() always set its path to *unchecked* slot, while
walk_up_tree() always think its path is set to *checked* slot.

Then this is indeed a problem.

> 
> If the last item was an inode item, yes, it was unchecked.
> Then walk_up_tree() will think the leaf is checked and walk up to
> upper node, process_one_leaf() in walk_down_tree() would skip to
> check next inode item. Which means, the inode item won't be checked.
> 
> Solution:
> After check_inode_item returns, if found path point to the last item
> of a leaf,

Would you please explain more about why last item makes a difference here?

>From previous statement, it looks like it's the difference in how
walk_up_tree() and check_inode_item() handles the path.
Not really related to last item.

Or did I miss something?

Thanks,
Qu

> decrease path slot manually, so walk_up_tree() will stay
> on the leaf.
> 
> Fixes: 5e2dc770471b ("btrfs-progs: check: skip shared node or leaf check for low_memory mode")
> Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
> ---
>  check/mode-lowmem.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/check/mode-lowmem.c b/check/mode-lowmem.c
> index 8fc9edab1d66..b6b33786d02b 100644
> --- a/check/mode-lowmem.c
> +++ b/check/mode-lowmem.c
> @@ -2612,6 +2612,18 @@ again:
>  	if (cur->start == cur_bytenr)
>  		goto again;
>  
> +	/*
> +	 * path may point at the last item(a inode item maybe) in a leaf.
> +	 * Without below lines, walk_up_tree() will skip the item which
> +	 * means all items related to the inode will never be checked.
> +	 * Decrease the slot manually, walk_up_tree won't skip to next node
> +	 * if it occurs.
> +	 */
> +	if (path->slots[0] + 1 >= btrfs_header_nritems(path->nodes[0])) {
> +		if (path->slots[0])
> +			path->slots[0]--;
> +	}
> +
>  	/*
>  	 * we have switched to another leaf, above nodes may
>  	 * have changed, here walk down the path, if a node
>