linux-ext4 - Re: [PATCH v7 4/5] dm-thin: Add REQ_OP

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Date:   Fri, 19 May 2023 11:23:12 -0400
From:   Mike Snitzer <snitzer@...nel.org>
To:     Sarthak Kukreti <sarthakkukreti@...omium.org>,
        Joe Thornber <ejt@...hat.com>
Cc:     dm-devel@...hat.com, linux-block@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Theodore Ts'o <tytso@....edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Jason Wang <jasowang@...hat.com>,
        Bart Van Assche <bvanassche@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Brian Foster <bfoster@...hat.com>,
        Alasdair Kergon <agk@...hat.com>
Subject: Re: [PATCH v7 4/5] dm-thin: Add REQ_OP_PROVISION support

On Thu, May 18 2023 at  6:33P -0400,
Sarthak Kukreti <sarthakkukreti@...omium.org> wrote:

> dm-thinpool uses the provision request to provision
> blocks for a dm-thin device. dm-thinpool currently does not
> pass through REQ_OP_PROVISION to underlying devices.
> 
> For shared blocks, provision requests will break sharing and copy the
> contents of the entire block. Additionally, if 'skip_block_zeroing'
> is not set, dm-thin will opt to zero out the entire range as a part
> of provisioning.
> 
> Signed-off-by: Sarthak Kukreti <sarthakkukreti@...omium.org>
> ---
>  drivers/md/dm-thin.c | 74 +++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 70 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> index 2b13c949bd72..f1b68b558cf0 100644
> --- a/drivers/md/dm-thin.c
> +++ b/drivers/md/dm-thin.c
> @@ -1245,8 +1247,8 @@ static int io_overlaps_block(struct pool *pool, struct bio *bio)
>  
>  static int io_overwrites_block(struct pool *pool, struct bio *bio)
>  {
> -	return (bio_data_dir(bio) == WRITE) &&
> -		io_overlaps_block(pool, bio);
> +	return (bio_data_dir(bio) == WRITE) && io_overlaps_block(pool, bio) &&
> +	       bio_op(bio) != REQ_OP_PROVISION;
>  }
>  
>  static void save_and_set_endio(struct bio *bio, bio_end_io_t **save,
> @@ -1394,6 +1396,9 @@ static void schedule_zero(struct thin_c *tc, dm_block_t virt_block,
>  	m->data_block = data_block;
>  	m->cell = cell;
>  
> +	if (bio && bio_op(bio) == REQ_OP_PROVISION)
> +		m->bio = bio;
> +
>  	/*
>  	 * If the whole block of data is being overwritten or we are not
>  	 * zeroing pre-existing data, we can issue the bio immediately.

This doesn't seem like the best way to address avoiding passdown of
provision bios (relying on process_prepared_mapping's implementation
that happens to do the right thing if m->bio set).  Doing so cascades
into relying on complete_overwrite_bio() happening to _not_ actually
being specific to "overwrite" bios.

I don't have a better suggestion yet but will look closer.  Just think
this needs to be formalized a bit more rather than it happening to
"just work".

Cc'ing Joe to see what he thinks too.  This is something we can clean
up with a follow-on patch though, so not a show-stopper for this
series.

Mike