linux-kernel - Re: [PATCH 3/5] dm: support retrieving struct dm_target from struct dm

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Date: Wed, 15 May 2024 11:42:04 -0400
From: Benjamin Marzinski <bmarzins@...hat.com>
To: Yang Yang <yang.yang@...o.com>
Cc: Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>,
        Mikulas Patocka <mpatocka@...hat.com>, dm-devel@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] dm: support retrieving struct dm_target from struct
 dm_dev

On Tue, May 14, 2024 at 05:04:42PM +0800, Yang Yang wrote:
> Add a list to the struct dm_dev structure to store the associated
> targets, while also allowing differentiation between different target
> types.

I still think this is more complex than it needs to be. If devices that
support flush_pass_around can guarantee that:

1. They will send a flush bio to all of their table devices
2. They are fine with another target sending the flush bio to their
   table devices

Then I don't see why we need the table devices to keep track of all the
different target types that are using them. Am I missing something here?

If we don't need to worry about sending a flush bio to a target of each
type that is using a table device, then all we need to do is call
__send_empty_flush_bios() for enough targets to cover all the table
devices. This seems a lot easier to track. We just need another flag in
dm_target, something like sends_pass_around_flush.

When a target calls dm_get_device(), if it adds a new table device to
t->devices, then it's the first target in this table to use that device.
If flush_pass_around is set for this target, then it also sets
sends_pass_around_flush. In __send_empty_flush() if the table has
flush_pass_around set, when you iterate through the devices, you only
call __send_empty_flush_bios() for the ones with sends_pass_around_flush
set.

Or am I overlooking something?

-Ben

> 
> Signed-off-by: Yang Yang <yang.yang@...o.com>
> ---
>  drivers/md/dm-table.c         | 36 +++++++++++++++++++++++++++++++++++
>  include/linux/device-mapper.h |  3 +++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index bd68af10afed..f6554590b7af 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -741,6 +741,8 @@ int dm_table_add_target(struct dm_table *t, const char *type,
>  	if (ti->flush_pass_around == 0)
>  		t->flush_pass_around = 0;
>  
> +	INIT_LIST_HEAD(&ti->list);
> +
>  	return 0;
>  
>   bad:
> @@ -2134,6 +2136,25 @@ void dm_table_postsuspend_targets(struct dm_table *t)
>  	suspend_targets(t, POSTSUSPEND);
>  }
>  
> +static int dm_link_dev_to_target(struct dm_target *ti, struct dm_dev *dev,
> +		sector_t start, sector_t len, void *data)
> +{
> +	struct list_head *targets = &dev->targets;
> +	struct dm_target *pti;
> +
> +	if (!list_empty(targets)) {
> +		list_for_each_entry(pti, targets, list) {
> +			if (pti->type == ti->type)
> +				return 0;
> +		}
> +	}
> +
> +	if (list_empty(&ti->list))
> +		list_add_tail(&ti->list, targets);
> +
> +	return 0;
> +}
> +
>  int dm_table_resume_targets(struct dm_table *t)
>  {
>  	unsigned int i;
> @@ -2162,6 +2183,21 @@ int dm_table_resume_targets(struct dm_table *t)
>  			ti->type->resume(ti);
>  	}
>  
> +	if (t->flush_pass_around) {
> +		struct list_head *devices = &t->devices;
> +		struct dm_dev_internal *dd;
> +
> +		list_for_each_entry(dd, devices, list)
> +			INIT_LIST_HEAD(&dd->dm_dev->targets);
> +
> +		for (i = 0; i < t->num_targets; i++) {
> +			struct dm_target *ti = dm_table_get_target(t, i);
> +
> +			if (ti->type->iterate_devices)
> +				ti->type->iterate_devices(ti, dm_link_dev_to_target, NULL);
> +		}
> +	}
> +
>  	return 0;
>  }
>  
> diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
> index 0893ff8c01b6..19e03f9b2589 100644
> --- a/include/linux/device-mapper.h
> +++ b/include/linux/device-mapper.h
> @@ -169,6 +169,7 @@ struct dm_dev {
>  	struct dax_device *dax_dev;
>  	blk_mode_t mode;
>  	char name[16];
> +	struct list_head targets;
>  };
>  
>  /*
> @@ -298,6 +299,8 @@ struct dm_target {
>  	struct dm_table *table;
>  	struct target_type *type;
>  
> +	struct list_head list;
> +
>  	/* target limits */
>  	sector_t begin;
>  	sector_t len;
> -- 
> 2.34.1
>