Re: [RFC PATCH 00/20] DCD: Remove support for sparse regions & add tags

[Alison Schofield <alison.schofield@intel.com>]

On Fri, Apr 10, 2026 at 06:22:55PM -0700, Anisa Su wrote:
> This RFC modifies Ira's DCD patchset to enforce tags and remove sparse DAX region
> support, which are the requirements that I've understood from the community
> meetings, with the end goal of FAMFS as the end user for DCD. Feedback would be
> greatly appreciated to let me know if this is on the right track? Or totally off
> the mark...
> 
> Everything is the same as before except:
> - extents must have tags (uuids)
> - 1 tag per region
> - regions must be contiguous (no more sparse regions)
> 
> To achieve this, the main thing is to change the relationship between
> cxl_dax_region : region_extent from 1 : many to 1 : 1. Each region_extent is
> comprised of 1+ contiguous device extents with the same tag. Contiguity
> is enforced by sorting device extents by DPA order. They're re-sorted by the
> original order in which they were sent for the response, which is required by
> the spec.
> 
> Once valid extents have been collected, it's passed as 1 contiguous capacity
> to the DAX layer via cxl_dax_region notify(). Once notified, the same region
> cannot be added to again, unless all extents are released.
> 
> For release: upon receiving a release event record, if the extent is within the
> bounds of any cxl_region, and it has the correct tag, then all extents in the
> region are released, so the "More" flag is still ignored. Not sure if this is the
> right way to do it but it was the simplest.
> 
> The changes to the DAX layer remain untouched, as all of this extra validation is done
> in the CXL layer. And since FAMFS already takes care of the devdax -> fsdev conversion,
> there was no need to add anything there.
> 
> Most of the series remains unchanged as I've tried not to make too many big changes
> right off the bat. Only the following commits were modified:
> - cxl/extent: Process dynamic partition events and realize region extents
> - dax/region: Create resources on DAX regions
> - cxl/region: Read existing extents on region creation
> 
> I've tacked on 1 commit at the end to change the driver type of DC regions from
> DAXDRV_KMEM_TYPE to DAXDRV_DEVICE_TYPE so it can be bound to the new fsdev driver.
> 
> Also, I've documented with more detail in the commit messages of the commits
> that were modified on what exactly was changed, so I hope that's clear.
> 
> ================================================================================
> Git History
> 
> This series is based on cxl-next, with base commit:
> 3939dba00f98 Merge branch 'for-7.1/cxl-misc' into cxl-for-next
> + bug fix: https://lore.kernel.org/linux-cxl/20260411011137.43545-1-anisa.su@samsung.com/T/#u
> 
> GH Branch: https://github.com/anisa-su993/anisa-linux-kernel/tree/dcd-rfc-04-10-26
> 
> It doesn't apply cleanly onto famfs-v9, although I have the version that's applied
> onto famfs-v9 here: https://github.com/anisa-su993/anisa-linux-kernel/tree/famfs-v9-dcd
> - famfs-v9 for reference: https://github.com/jagalactic/linux/tree/famfs-v9
> 
> I've tested the current series without famfs as well as the series applied on
> famfs-v9 with famfs.
> ================================================================================
> Testing:

Where's the CXl unit testing...

big skip


>   tools/testing/cxl: Make event logs dynamic
>   tools/testing/cxl: Add DC Regions to mock mem data

Hi Anisa,

I'm inserting here and referencing the last 2 patches above
that add cxl/test support for DCD testing.

Let's use it! We'd really like to see the ndctl patches sync'd up
with this kernel set. That would give us a CXL Unit test 'cxl-dcd.sh'
to run on each revision. 

This is one link, perhaps the last to the ndctl set, but I think you
may have posted something long ago too:
https://lore.kernel.org/nvdimm/20250413-dcd-region2-v5-0-fbd753a2e0e8@intel.com/

-- Alison