linux-kernel - Re: [PATCH v3 22/25] cxl/region: Read existing extents on region creation

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Message-ID: <20240827151952.0000429b@Huawei.com>
Date: Tue, 27 Aug 2024 15:19:52 +0100
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>, 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 22/25] cxl/region: Read existing extents on region
 creation

On Fri, 16 Aug 2024 09:44:30 -0500
ira.weiny@...el.com wrote:

> From: Navneet Singh <navneet.singh@...el.com>
> 
> 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.
> 
> 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.

Ah. So the earlier code to just eat duplicates was to handle this race.
Add a comment there perhaps so people like me get less confused :)

Jonathan

> 
> 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>

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