linux-kernel - Re: [PATCH v4 21/28] cxl/extent: Process DCD events and realize region extents

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Message-ID: <6716a165823b7_8cb1729437@iweiny-mobl.notmuch>
Date: Mon, 21 Oct 2024 13:45:57 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>, Ira Weiny
	<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-btrfs@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <nvdimm@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 21/28] cxl/extent: Process DCD events and realize
 region extents

Jonathan Cameron wrote:
> On Thu, 17 Oct 2024 16:39:57 -0500
> Ira Weiny <ira.weiny@...el.com> wrote:
> 
> > Jonathan Cameron wrote:
> > > On Mon, 07 Oct 2024 18:16:27 -0500
> > > ira.weiny@...el.com wrote:
> > >   

[snip]

> > > > 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.   
> > > 
> > > Maybe scream about this a little.  AFAIK that happening is a device
> > > bug.  
> > 
> > Agreed but because of the 2nd purpose this is difficult to scream about because
> > this situation can come up in normal operation.  Here is the scenario:
> > 
> > 1) Device has 2 DCD partitions active, A and B
> > 2) Host crashes
> > 3) Region X is created on A
> > 4) Region Y is created on B
> > 5) Region Y scans for extents
> > 6) Region X surfaces a new extent while Y is scanning
> > 7) Gen number changes due to new extent in X
> > 8) Region Y rescans for existing extents and sees duplicates.
> > 
> > These duplicates need to be ignored without signaling an error.
> Hmm. If we can know that path is the trigger (should be able to
> as it's a scan after a gen number change), can we just muffle the
> screams on that path? (Halloween is close, the analogies will get
> ever worse :)

Ok yea since this would be a device error we should do something here.  But the
code is going to be somewhat convoluted to print an error whenever this
happens.

What if we make this a warning and change the rescan debug message to a warning
as well?  This would allow enough bread crumbs to determine if a device is
failing without a lot of extra code to alter print messages on the fly?

Ira