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SubjectReply [PATCH] dm-verity: unnecessary data blocks that need not read hash blocks
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hey Eric:

On Wed, Dec 11, 2019 at 11:32:40AM +0800, zhou xianrong wrote:
> From: "xianrong.zhou" <xianrong.zhou@transsion.com>
>
> 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@transsion.com>
> Signed-off-by: yuanjiong.gao <yuanjiong.gao@transsion.com>
> Tested-by: ruxian.feng <ruxian.feng@transsion.com>
> ---
> 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
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