linux-kernel - Re: [PATCH RFC v2 10/18] cxl/mem: Handle DCD add and release capacity events.

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Date:   Tue, 29 Aug 2023 16:59:30 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Ira Weiny <ira.weiny@...el.com>
CC:     Dan Williams <dan.j.williams@...el.com>,
        Navneet Singh <navneet.singh@...el.com>,
        Fan Ni <fan.ni@...sung.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 10/18] cxl/mem: Handle DCD add and release
 capacity events.

On Mon, 28 Aug 2023 22:21:01 -0700
Ira Weiny <ira.weiny@...el.com> wrote:

> A Dynamic Capacity Device (DCD) utilizes events to signal the host about
> the changes to the allocation of Dynamic Capacity (DC) extents. The
> device communicates the state of DC extents through an extent list that
> describes the starting DPA, length, and meta data of the blocks the host
> can access.
> 
> Process the dynamic capacity add and release events.  The addition or
> removal of extents can occur at any time.  Adding asynchronous memory is
> straight forward.  Also remember the host is under no obligation to
> respond to a release event until it is done with the memory.  Introduce
> extent kref's to handle the delay of extent release.
> 
> In the case of a force removal, access to the memory will fail and may
> cause a crash.  However, the extent tracking object is preserved for the
> region to safely tear down as long as the memory is not accessed.
> 
> Signed-off-by: Navneet Singh <navneet.singh@...el.com>
> Co-developed-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
Minor stuff inline.


> +static int cxl_prepare_ext_list(struct cxl_mbox_dc_response **res,
> +				int *n, struct range *extent)
> +{
> +	struct cxl_mbox_dc_response *dc_res;
> +	unsigned int size;
> +
> +	if (!extent)
> +		size = struct_size(dc_res, extent_list, 0);

This is confusing as if you did have *n > 0 I'd kind of expect
this to just not extend the list rather than shortening it.
Now I guess that never happens, but locally it looks odd.

Maybe just handle that case in a separate function as it doesn't
share much code with the case where there is an extent and I would
assume we always know at the caller which one we want.


> +	else
> +		size = struct_size(dc_res, extent_list, *n + 1);

Might be clearer with a local variable for the number of extents.

extents_count = *n;

if (extent)
	extents_count++;

size = struct_size(dc_res, extent_list, extents_count);

Though I'm not sure that really helps.  Maybe this will just need
to be a little confusing :)

> +
> +	dc_res = krealloc(*res, size, GFP_KERNEL);
> +	if (!dc_res)
> +		return -ENOMEM;
> +
> +	if (extent) {
> +		dc_res->extent_list[*n].dpa_start = cpu_to_le64(extent->start);
> +		memset(dc_res->extent_list[*n].reserved, 0, 8);
> +		dc_res->extent_list[*n].length = cpu_to_le64(range_len(extent));
> +		(*n)++;
> +	}
> +
> +	*res = dc_res;
> +	return 0;
> +}

> +
> +/* Returns 0 if the event was handled successfully. */
> +static int cxl_handle_dcd_event_records(struct cxl_memdev_state *mds,
> +					struct cxl_event_record_raw *rec)
> +{
> +	struct dcd_event_dyn_cap *record = (struct dcd_event_dyn_cap *)rec;
> +	uuid_t *id = &rec->hdr.id;
> +	int rc;
> +
> +	if (!uuid_equal(id, &dc_event_uuid))
> +		return -EINVAL;
> +
> +	switch (record->data.event_type) {
> +	case DCD_ADD_CAPACITY:
> +		rc = cxl_handle_dcd_add_event(mds, &record->data.extent);
> +		break;

I guess it might not be consistent with local style...
		return cxl_handle_dcd_add_event()  etc

> +	case DCD_RELEASE_CAPACITY:
> +        case DCD_FORCED_CAPACITY_RELEASE:
> +		rc = cxl_handle_dcd_release_event(mds, &record->data.extent);
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return rc;
> +}
> +