Re: [PATCH] Resource: fix region_intersects() for CXL memory

[Dan Williams <dan.j.williams@intel.com>]

[ add David ]

Andrew Morton wrote:
> On Fri, 16 Aug 2024 10:07:23 +0800 Huang Ying <ying.huang@intel.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.