linux-kernel - Re: [PATCH v3 18/25] cxl/extent: Process DCD events and realize region extents

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Message-ID: <ZtoGzNhrCOPvrnGV@leg>
Date: Thu, 5 Sep 2024 12:30:20 -0700
From: Fan Ni <nifan.cxl@...il.com>
To: ira.weiny@...el.com
Cc: Dave Jiang <dave.jiang@...el.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Navneet Singh <navneet.singh@...el.com>, Chris Mason <clm@...com>,
	Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	linux-btrfs@...r.kernel.org, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	nvdimm@...ts.linux.dev
Subject: Re: [PATCH v3 18/25] cxl/extent: Process DCD events and realize
 region extents

On Fri, Aug 16, 2024 at 09:44:26AM -0500, ira.weiny@...el.com wrote:
> From: Navneet Singh <navneet.singh@...el.com>
> 
> A dynamic capacity device (DCD) sends events to signal the host for
> changes in the availability of Dynamic Capacity (DC) memory.  These
> events contain extents describing a DPA range and meta data for memory
> to be added or removed.  Events may be sent from the device at any time.
> 
> Three types of events can be signaled, Add, Release, and Force Release.
> 
> On add, the host may accept or reject the memory being offered.  If no
> region exists, or the extent is invalid, the extent should be rejected.
> Add extent events may be grouped by a 'more' bit which indicates those
> extents should be processed as a group.
> 
> On remove, the host can delay the response until the host is safely not
> using the memory.  If no region exists the release can be sent
> immediately.  The host may also release extents (or partial extents) at
> any time.  Thus the 'more' bit grouping of release events is of less
> value and can be ignored in favor of sending multiple release capacity
> responses for groups of release events.
> 
> Force removal is intended as a mechanism between the FM and the device
> and intended only when the host is unresponsive, out of sync, or
> otherwise broken.  Purposely ignore force removal events.
> 
> Regions are made up of one or more devices which may be surfacing memory
> to the host.  Once all devices in a region have surfaced an extent the
> region can expose a corresponding extent for the user to consume.
> Without interleaving a device extent forms a 1:1 relationship with the
> region extent.  Immediately surface a region extent upon getting a
> device extent.
> 
> Per the specification the device is allowed to offer or remove extents
> at any time.  However, anticipated use cases can expect extents to be
> offered, accepted, and removed in well defined chunks.
> 
> Simplify extent tracking with the following restrictions.
> 
> 	1) Flag for removal any extent which overlaps a requested
> 	   release range.
> 	2) Refuse the offer of extents which overlap already accepted
> 	   memory ranges.
> 	3) Accept again a range which has already been accepted by the
> 	   host.  (It is likely the device has an error because it
> 	   should already know that this range was accepted.  But from
> 	   the host point of view it is safe to acknowledge that
> 	   acceptance again.)
> 
> Management of the region extent devices must be synchronized with
> potential uses of the memory within the DAX layer.  Create region extent
> devices as children of the cxl_dax_region device such that the DAX
> region driver can co-drive them and synchronize with the DAX layer.
> Synchronization and management is handled in a subsequent patch.
> 
> Process DCD events and create region devices.
> 
> 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>
> 
One more minor inline.
> +static int cxl_validate_extent(struct cxl_memdev_state *mds,
> +			       struct cxl_extent *extent)
> +{
> +	u64 start = le64_to_cpu(extent->start_dpa);
> +	u64 length = le64_to_cpu(extent->length);
> +	struct device *dev = mds->cxlds.dev;
> +
> +	struct range ext_range = (struct range){
> +		.start = start,
> +		.end = start + length - 1,
> +	};
> +
> +	if (le16_to_cpu(extent->shared_extn_seq) != 0) {
> +		dev_err_ratelimited(dev,
> +				    "DC extent DPA %par (%*phC) can not be shared\n",
> +				    &ext_range.start, CXL_EXTENT_TAG_LEN,
> +				    extent->tag);
> +		return -ENXIO;
> +	}
> +
> +	/* Extents must not cross DC region boundary's */
> +	for (int i = 0; i < mds->nr_dc_region; i++) {
> +		struct cxl_dc_region_info *dcr = &mds->dc_region[i];
> +		struct range region_range = (struct range) {
> +			.start = dcr->base,
> +			.end = dcr->base + dcr->decode_len - 1,
> +		};
> +
> +		if (range_contains(&region_range, &ext_range)) {
> +			dev_dbg(dev, "DC extent DPA %par (DCR:%d:%#llx)(%*phC)\n",
> +				&ext_range, i, start - dcr->base,
> +				CXL_EXTENT_TAG_LEN, extent->tag);
> +			return 0;
> +		}
> +	}

For extent validation, we may need to ensure its size is not 0 based on the spec.
Noted that during testing, do not see issue for the case as 0-sized
extents will be rejected when trying to add even though it passes the
validation.

Fan