linux-kernel - Re: [PATCH] Resource: fix region

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Message-ID: <66bede74cb96f_1c18294ce@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Thu, 15 Aug 2024 22:07:00 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Huang Ying
	<ying.huang@...el.com>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<linux-cxl@...r.kernel.org>, Dan Williams <dan.j.williams@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Alistair Popple
	<apopple@...dia.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Baoquan He <bhe@...hat.com>,
	<david@...hat.com>
Subject: Re: [PATCH] Resource: fix region_intersects() for CXL memory

[ add David ]

Andrew Morton wrote:
> On Fri, 16 Aug 2024 10:07:23 +0800 Huang Ying <ying.huang@...el.com> wrote:
> 
> > On a system with CXL memory installed, the resource tree (/proc/iomem)
> > related to CXL memory looks like something as follows.
> > 
> > 490000000-50fffffff : CXL Window 0
> >   490000000-50fffffff : region0
> >     490000000-50fffffff : dax0.0
> >       490000000-50fffffff : System RAM (kmem)
> > 
> > When the following command line is run to try writing some memory in
> > CXL memory range,
> > 
> >  $ dd if=data of=/dev/mem bs=1k seek=19136512 count=1
> >  dd: error writing '/dev/mem': Bad address
> >  1+0 records in
> >  0+0 records out
> >  0 bytes copied, 0.0283507 s, 0.0 kB/s
> > 
> > the command fails as expected.  However, the error code is wrong.  It
> > should be "Operation not permitted" instead of "Bad address".  And,
> > the following warning is reported in kernel log.
> > 
> >  ioremap on RAM at 0x0000000490000000 - 0x0000000490000fff
> >  WARNING: CPU: 2 PID: 416 at arch/x86/mm/ioremap.c:216 __ioremap_caller.constprop.0+0x131/0x35d
> > ...
> >
> 
> Presumably we want to fix earlier kernels?  If so, are you able to
> identify a suitable Fixes: target?  Possibly 974854ab0728 ("cxl/acpi:
> Track CXL resources in iomem_resource")?

At least that commit, but I think this problem potentially goes back
farther to:

c221c0b0308f device-dax: "Hotplug" persistent memory for use like normal RAM

...because that started the era of "System RAM" as a non-top-level
resource.

David did a bunch of work to fix this back in:

97f61c8f44ec kernel/resource: make walk_system_ram_res() find all busy IORESOURCE_SYSTEM_RAM resources

..but the fallout in region_intersects() was missed.