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

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Date: Thu, 16 May 2024 09:55:53 +0800
From: YangYang <yang.yang@...o.com>
To: Benjamin Marzinski <bmarzins@...hat.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 2024/5/15 23:42, Benjamin Marzinski wrote:
> 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?

I attempted to enhance this solution to support additional target types,
such as those with num_flush_bios greater than 1.

> 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?

If I understand correctly, you are suggesting to iterate through all the
targets, handling those with sends_pass_around_flush set, and skipping
those where sends_pass_around_flush is not set. I believe this approach
may result in some CPU wastage.

   for i in {0..1023}; do
     echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
   done | sudo dmsetup create example

In this specific scenario, a single iteration of the loop is all that
is needed.

> 
> -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
>>
>