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

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Message-ID: <20241018100909.00001ec2@Huawei.com>
Date: Fri, 18 Oct 2024 10:09:09 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: 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

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:
> >   
> > > 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.  
> > 
> > True today - I think that would be an error for shared extents
> > though as they need to be released in one go.  We can deal with
> > that when it matters.  
> > 
> > 
> > Mind you patch seems to try to handle more bit anyway, so maybe just
> > remove that discussion from this description?  
> 
> It only handles more bit response on ADD because on RELEASE the count is always
> 1.
> 
> 
> +       if (cxl_send_dc_response(mds, CXL_MBOX_OP_RELEASE_DC, &extent_list, 1)) 
> +               dev_dbg(dev, "Failed to release [range 0x%016llx-0x%016llx]\n", 
> +                       range->start, range->end);                              
> 
> 
> For shared; a flag will need to be added to the extents and additional logic to
> group these extents for checking use etc.  
> 
> I agree, we need to handle that later on and get this basic support in.  For
> now I think my comments are correct WRT the sending of release responses.
> 
> > > 
> > > 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 :)

Jonathan