Reply [PATCH] dm-verity: unnecessary data blocks that need not read hash blocks

[xianrong.zhou(周先荣)]

hey Eric:

On Wed, Dec 11, 2019 at 11:32:40AM +0800, zhou xianrong wrote:
> From: "xianrong.zhou" <xianrong.zhou@xxxxxxxxxxxxx>
> 
> If check_at_most_once enabled, just like verity work the prefetching 
> work should check for data block bitmap firstly before reading hash 
> block as well. Skip bit-set data blocks from both ends of data block 
> range by testing the validated bitmap. This can reduce the amounts of 
> data blocks which need to read hash blocks.
> 
> Launching 91 apps every 15s and repeat 21 rounds on Android Q.
> In prefetching work we can let only 2602/360312 = 0.72% data blocks 
> really need to read hash blocks.
> 
> But the reduced data blocks range would be enlarged again by 
> dm_verity_prefetch_cluster later.
> 
> Signed-off-by: xianrong.zhou <xianrong.zhou@xxxxxxxxxxxxx>
> Signed-off-by: yuanjiong.gao <yuanjiong.gao@xxxxxxxxxxxxx>
> Tested-by: ruxian.feng <ruxian.feng@xxxxxxxxxxxxx>
> ---
>  drivers/md/dm-verity-target.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/md/dm-verity-target.c 
> b/drivers/md/dm-verity-target.c index 4fb33e7562c5..7b8eb754c0b6 
> 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -581,6 +581,22 @@ static void verity_prefetch_io(struct work_struct *work)
>  	struct dm_verity *v = pw->v;
>  	int i;
>  
> +	if (v->validated_blocks) {
> +		while (pw->n_blocks) {
> +			if (unlikely(!test_bit(pw->block, v->validated_blocks)))
> +				break;
> +			pw->block++;
> +			pw->n_blocks--;
> +		}
> +		while (pw->n_blocks) {
> +			if (unlikely(!test_bit(pw->block + pw->n_blocks - 1,
> +				v->validated_blocks)))
> +				break;
> +			pw->n_blocks--;
> +		}
> +		if (!pw->n_blocks)
> +			return;
> +	}

This is a good idea, but shouldn't this logic go in verity_submit_prefetch() prior to the struct dm_verity_prefetch_work being allocated?  Then if no prefeching is needed, allocating and scheduling the work object can be skipped.

Eric, Do you mean it is more suitable in dm_bufio_prefetch which is called on different paths even though prefeching is disabled ?

Also note that you're currently leaking the work object with the early return.	

Right! I leaked this. always so. Thanks!!!

- Eric