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

[Eric Biggers]

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.

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

- Eric