linux-kernel - Re: [PATCH v6 20/27] cxl/extent: Process DCD events and realize region extents

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Message-ID: <20241107112950.00000384@Huawei.com>
Date: Thu, 7 Nov 2024 11:29:50 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: <ira.weiny@...el.com>
CC: Dave Jiang <dave.jiang@...el.com>, Fan Ni <fan.ni@...sung.com>, "Navneet
 Singh" <navneet.singh@...el.com>, 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-cxl@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 20/27] cxl/extent: Process DCD events and realize
 region extents

On Tue, 05 Nov 2024 12:38:42 -0600
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.  Eating duplicates serves three purposes.  First, this
> 	   simplifies the code if the device should get out of sync with
> 	   the host.  And it should be safe to acknowledge the extent
> 	   again.  Second, this simplifies the code to process existing
> 	   extents if the extent list should change while the extent
> 	   list is being read.  Third, duplicates for a given region
> 	   which are seen during a race between the hardware surfacing
> 	   an extent and the cxl dax driver scanning for existing
> 	   extents will be ignored.
> 
> 	   NOTE: Processing existing extents is done in a later patch.
> 
> 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.
> 
> Tag support within the DAX layer is not yet supported.  To maintain
> compatibility legacy DAX/region processing only tags with a value of 0
> are allowed.  This defines existing DAX devices as having a 0 tag which
> makes the most logical sense as a default.
> 
> Process DCD events and create region devices.
> 
> Signed-off-by: Navneet Singh <navneet.singh@...el.com>
> Reviewed-by: Dave Jiang <dave.jiang@...el.com>
> Co-developed-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> 
I gave this on v5, but meh here it is again (obviously subject to
Li Ming's fix which maybe can just get rolled in whilst applying)

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>