Re: [PATCH v9 16/19] cxl/region: Read existing extents on region creation

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Sun, 13 Apr 2025 17:52:24 -0500
Ira Weiny <ira.weiny@intel.com> wrote:

> Dynamic capacity device extents may be left in an accepted state on a
> device due to an unexpected host crash.  In this case it is expected
> that the creation of a new region on top of a DC partition can read
> those extents and surface them for continued use.
> 
> Once all endpoint decoders are part of a region and the region is being
> realized, a read of the 'devices extent list' can reveal these
> previously accepted extents.
> 
> CXL r3.1 specifies the mailbox call Get Dynamic Capacity Extent List for
> this purpose.  The call returns all the extents for all dynamic capacity
> partitions.  If the fabric manager is adding extents to any DCD
> partition, the extent list for the recovered region may change.  In this
> case the query must retry.  Upon retry the query could encounter extents
> which were accepted on a previous list query.  Adding such extents is
> ignored without error because they are entirely within a previous
> accepted extent.  Instead warn on this case to allow for differentiating
> bad devices from this normal condition.
> 
> Latch any errors to be bubbled up to ensure notification to the user
> even if individual errors are rate limited or otherwise ignored.
> 
> The scan for existing extents races with the dax_cxl driver.  This is
> synchronized through the region device lock.  Extents which are found
> after the driver has loaded will surface through the normal notification
> path while extents seen prior to the driver are read during driver load.
> 
> Based on an original patch by Navneet Singh.
> 
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Reviewed-by: Fan Ni <fan.ni@samsung.com>
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>

A couple of minor things noticed on taking another look.

> diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
> index de01c6684530..8af3a4173b99 100644
> --- a/drivers/cxl/core/mbox.c
> +++ b/drivers/cxl/core/mbox.c
> @@ -1737,6 +1737,115 @@ int cxl_dev_dc_identify(struct cxl_mailbox *mbox,
>  }
>  EXPORT_SYMBOL_NS_GPL(cxl_dev_dc_identify, "CXL");
>  
> +/* Return -EAGAIN if the extent list changes while reading */
> +static int __cxl_process_extent_list(struct cxl_endpoint_decoder *cxled)
> +{
> +	u32 current_index, total_read, total_expected, initial_gen_num;
> +	struct cxl_memdev_state *mds = cxled_to_mds(cxled);
> +	struct cxl_mailbox *cxl_mbox = &mds->cxlds.cxl_mbox;
> +	struct device *dev = mds->cxlds.dev;
> +	struct cxl_mbox_cmd mbox_cmd;
> +	u32 max_extent_count;
> +	int latched_rc = 0;
> +	bool first = true;
> +
> +	struct cxl_mbox_get_extent_out *extents __free(kvfree) =
> +				kvmalloc(cxl_mbox->payload_size, GFP_KERNEL);
> +	if (!extents)
> +		return -ENOMEM;
> +
> +	total_read = 0;
> +	current_index = 0;
> +	total_expected = 0;
> +	max_extent_count = (cxl_mbox->payload_size - sizeof(*extents)) /
> +				sizeof(struct cxl_extent);
> +	do {
> +		u32 nr_returned, current_total, current_gen_num;
> +		struct cxl_mbox_get_extent_in get_extent;
> +		int rc;
> +
> +		get_extent = (struct cxl_mbox_get_extent_in) {
> +			.extent_cnt = cpu_to_le32(max(max_extent_count,
> +						  total_expected - current_index)),
> +			.start_extent_index = cpu_to_le32(current_index),
> +		};
> +
> +		mbox_cmd = (struct cxl_mbox_cmd) {
> +			.opcode = CXL_MBOX_OP_GET_DC_EXTENT_LIST,
> +			.payload_in = &get_extent,
> +			.size_in = sizeof(get_extent),
> +			.size_out = cxl_mbox->payload_size,
> +			.payload_out = extents,
> +			.min_out = 1,

Similar to earlier comment (I might well have forgotten how this works) but
why not 16 which is what I think we should get even if no extents.

> +		};
> +
> +		rc = cxl_internal_send_cmd(cxl_mbox, &mbox_cmd);
> +		if (rc < 0)
> +			return rc;
> +
> +		/* Save initial data */
> +		if (first) {
> +			total_expected = le32_to_cpu(extents->total_extent_count);
> +			initial_gen_num = le32_to_cpu(extents->generation_num);
> +			first = false;
> +		}
> +
> +		nr_returned = le32_to_cpu(extents->returned_extent_count);
> +		total_read += nr_returned;
> +		current_total = le32_to_cpu(extents->total_extent_count);
> +		current_gen_num = le32_to_cpu(extents->generation_num);
> +
> +		dev_dbg(dev, "Got extent list %d-%d of %d generation Num:%d\n",
> +			current_index, total_read - 1, current_total, current_gen_num);
> +
> +		if (current_gen_num != initial_gen_num || total_expected != current_total) {
> +			dev_warn(dev, "Extent list change detected; gen %u != %u : cnt %u != %u\n",
> +				 current_gen_num, initial_gen_num,
> +				 total_expected, current_total);
> +			return -EAGAIN;
> +		}
> +
> +		for (int i = 0; i < nr_returned ; i++) {
> +			struct cxl_extent *extent = &extents->extent[i];
> +
> +			dev_dbg(dev, "Processing extent %d/%d\n",
> +				current_index + i, total_expected);
> +
> +			rc = validate_add_extent(mds, extent);
> +			if (rc)
> +				latched_rc = rc;
> +		}
> +
> +		current_index += nr_returned;
> +	} while (total_expected > total_read);
> +
> +	return latched_rc;
> +}

> +/*
> + * Get Dynamic Capacity Extent List; Output Payload
> + * CXL rev 3.1 section 8.2.9.9.9.2; Table 8-167
> + */
> +struct cxl_mbox_get_extent_out {
> +	__le32 returned_extent_count;
> +	__le32 total_extent_count;
> +	__le32 generation_num;
> +	u8 rsvd[4];
> +	struct cxl_extent extent[];

Throw some counted_by magic at this?

> +} __packed;
> +
>  struct cxl_mbox_get_supported_logs {
>  	__le16 entries;
>  	u8 rsvd[6];
>