Re: [RFC PATCH 0/4] cxl/extent: Enable and validate multi-extent DCDs

[Anisa Su]

On Thu, Apr 23, 2026 at 11:51:12PM +0000, John Groves wrote:
> From: John Groves <john@xxxxxxxxxx>
> 
> This series fixes correctness issues in the DCD (Dynamic Capacity
> Device) add-capacity pipeline so that multi-tag allocations, sharable
> CDAT regions, and cross-partition checks work end-to-end.
> 
[snip] 
Hi John,

I've already squashed your branches to a GH branch so they're just hanging out
until we get some more feedback.
But I do have some food for thought relating to the DAX layer changes that would
be required to make this patchset function the way it's intended:

Currently DAX device creation is not tag-aware. This patchset only affects the
conditions of when we choose to accept/reject extents. But accepted extents just
hang out under the region until a DAX device is created and
claims resources on the region.

Currently, doing daxctl create-device on a dynamic_ram_a type region does 2
things:

daxctl create-device -r region0 -s requested_size
1. create a 0-sized dev_dax seed:
	- create_store()
	- extents must have been added to the region already, otherwise returns
	  -ENOSPC
		avail = dax_region_avail_size(dax_region);
		if (avail == 0)
			rc = -ENOSPC;
	- creates a 0-sized DAX device seed (Sparse DAX region devices must be
	  created initially with 0 size)

2. resize the empty dev_dax to requested_size or the size of the region if -s
not specified
	- size_store(to_alloc = requested_size ? requested_size :
	  dax_region_avail_size(dax_region))
	- dev_dax_resize()
	- iterates over each child resource of the region to find unused
	  resources and claims them up to min(available_size, to_alloc)
	- for sparse regions, each child resource = 1 region extent
		- *note*: in Ira's original patchset, each region_extent = 1
		  device extent.
		  Thus each resource under the resource tree of the dax_region
		  corresponds to 1 device extent
		- In the current version: 1 region_extent --> 1 tagged
		  allocation (1+ extents w/the same tag)
		- this should also be cleared up

If we want each DAX device to correspond to a specific tag (tag-aware DAX device
creation):
1. Add tag to struct dax_resource
/**
 * struct dax_resource - For sparse regions; an active resource
 * @region: dax_region this resources is in
 * @res: resource
 * @use_cnt: count the number of uses of this resource
 *
 * Changes to the dax_region and the dax_resources within it are protected by
 * dax_region_rwsem
 *
 * dax_resource's are not intended to be used outside the dax layer.
 */
struct dax_resource {
	struct dax_region *region;
	struct resource *res;
	unsigned int use_cnt;
+	uuid_t tag;
};

2. In the resizing operation:
	- only allow dax device to claim a resource if it has the right tag
	- all resources with that tag must be claimed by that device
	- currently, the logic allows for a DAX device partially claim an extent 
	  if requested_size_to_alloc < size_of_free_extent(s); would need to be
	  disallowed for tagged extents

3. ? New sysfs interface to allow for something like "echo [tag_abcd] >
/sys/.../daxX.Y/tag"

4. If we allow non-tagged extents (as this patchset allows):
	- can non-tagged extents be partially claimed by a DAX device that
	  doesn't have a specified tag?

Ira's original patchset introduced minimal changes to DAX logic. Introducing
tag-awareness to
DAX resize logic may require adding more conditional statements, 
especially if we allow both NULL and non-NULL tags (but there might be a nice
way to do it;
This is just my initial impression, I haven't tried implementing it, and I could
certainly look into it more)

TLDR; some topics for discussion:
1. region_extent to device extent relationship
	- Ira said w/o interleaving, it should remain 1:1, not 1:many for
	  grouping device extents by tag

2. tag-aware DAX device creation
	- changes needed & their implications
	- what to do with non-tagged extents

See you at LSFMM :)

Thanks,
Anisa
 
> Signed-off-by: John Groves <John@xxxxxxxxxx>
> 
> John Groves (4):
>   cxl/extent: Promote cxlr_dax->region_extent to an xarray
>   cxl/extent: Fix DCD add-capacity: per-tag assembly, ordering, and
>     integrity
>   cxl/extent: Support extents in sharable CDAT regions
>   cxl/extent: Reject tagged extents that span DC partitions
> 
>  drivers/cxl/core/extent.c |  93 +++---
>  drivers/cxl/core/mbox.c   | 628 +++++++++++++++++++++++++++++++-------
>  drivers/cxl/core/region.c |   2 +
>  drivers/cxl/cxl.h         |   9 +-
>  include/cxl/event.h       |   1 -
>  5 files changed, 584 insertions(+), 149 deletions(-)
> 
> -- 
> 2.53.0
> 
>