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

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Message-ID: <20240827130829.00004660@Huawei.com>
Date: Tue, 27 Aug 2024 13:08:29 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Fan Ni <nifan.cxl@...il.com>
CC: <ira.weiny@...el.com>, Dave Jiang <dave.jiang@...el.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, 23 Aug 2024 14:32:32 -0700
Fan Ni <nifan.cxl@...il.com> wrote:

> 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 minor change inline.
Hi Fan,

Crop please.  I scanned past it 3 times when scrolling without noticing
what you'd actually commented on.

> > +/* See CXL 3.0 8.2.9.2.1.5 */  
> 
> Update the reference to reflect CXL 3.1.
> 
> Fan
>