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

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Message-ID: <20200107031429.GA705@sol.localdomain>
Date:   Mon, 6 Jan 2020 19:14:29 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     zhou_xianrong <zhou_xianrong@...u.com>
Cc:     dm-devel@...hat.com, linux-kernel@...r.kernel.org, agk@...hat.com,
        snitzer@...hat.com, wanbin.wang@...nssion.com,
        haizhou.song@...nssion.com,
        "xianrong.zhou" <xianrong.zhou@...nssion.com>,
        "yuanjiong . gao" <yuanjiong.gao@...nssion.com>,
        "ruxian . feng" <ruxian.feng@...nssion.com>
Subject: Re: [PATCH] dm-verity:unnecessary data blocks that need not read
 hash blocks

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;
	}