Re: [PATCH 3/5] dm: support retrieving struct dm_target from struct dm_dev

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 15 May 2024, 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?
> 
> 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

Yes, I agree that it is complex.

I reworked the patch, I'm testing it now and I'll send it when it passes 
the tests.

Mikulas