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

[Eric Biggers]

On Tue, Jan 07, 2020 at 10:48:43AM +0800, zhou_xianrong wrote:

> Subject: [PATCH] dm-verity:unnecessary data blocks that need not read

Please use a proper commit subject.  It should begin with "dm verity: " and use
the imperative tense to describe the change, e.g.

	dm verity: don't prefetch hash blocks for already-verified data

Also this should have been "PATCH v2", not simply PATCH, since you already sent
out a previous version.

You also sent out multiple copies of this email for some reason, so I just chose
one arbitrarily to reply to...

> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index 4fb33e7..4127711 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -611,8 +611,27 @@ static void verity_prefetch_io(struct work_struct *work)
>  
>  static void verity_submit_prefetch(struct dm_verity *v, struct dm_verity_io *io)
>  {
> +	sector_t block = io->block;
> +	unsigned int n_blocks = io->n_blocks;
>  	struct dm_verity_prefetch_work *pw;
>  
> +	if (v->validated_blocks) {
> +		while (n_blocks) {
> +			if (unlikely(!test_bit(block, v->validated_blocks)))
> +				break;
> +			block++;
> +			n_blocks--;
> +		}
> +		while (n_blocks) {
> +			if (unlikely(!test_bit(block + n_blocks - 1,
> +				v->validated_blocks)))
> +				break;
> +			n_blocks--;
> +		}
> +		if (!n_blocks)
> +			return;
> +	}

This looks fine now, though it's a bit more verbose than necessary, and I don't
think unlikely() will make any difference here.  Consider simplifying it to:

	if (v->validated_blocks) {
		while (n_blocks && test_bit(block, v->validated_blocks)) {
			block++;
			n_blocks--;
		}
		while (n_blocks && test_bit(block + n_blocks - 1,
					    v->validated_blocks))
			n_blocks--;
		if (!n_blocks)
			return;
	}