Re: [PATCH v3 5/5] crash: export crashkernel CMA reservation to userspace

[Baoquan he <bhe@redhat.com>]

On 11/10/25 at 02:09pm, Sourabh Jain wrote:
> 
> 
> On 10/11/25 12:38, Baoquan he wrote:
> > On 11/10/25 at 10:01am, Sourabh Jain wrote:
> > > Add a sysfs entry /sys/kernel/kexec/crash_cma_ranges to expose all
> > > CMA crashkernel ranges.
> > I am not against this way. While wondering if it's more appropriate to
> > export them into iomem_resource just like crashk_res and crashk_low_res
> > doing.
> 
> Handling conflict is challenging. Hence we don't export crashk_res and
> crashk_low_res to iomem on powerpc. Checkout [1]
> 
> And I think conflicts can occur regardless of the order in which System RAM
> and
> Crash CMA ranges are added to iomem.
> 
> [1] https://lore.kernel.org/all/20251016142831.144515-1-sourabhjain@linux.ibm.com/

Then I would suggest you add this reason and the link into patch log
to keep a record. One day people may post patch to 'optimize' this.

> 
> > 
> > > This allows userspace tools configuring kdump to determine how much
> > > memory is reserved for crashkernel. If CMA is used, tools can warn
> > > users when attempting to capture user pages with CMA reservation.
> > > 
> > > The new sysfs hold the CMA ranges in below format:
> > > 
> > > cat /sys/kernel/kexec/crash_cma_ranges
> > > 100000000-10c7fffff
> > > 
> > > Cc: Aditya Gupta <adityag@linux.ibm.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Baoquan he <bhe@redhat.com>
> > > Cc: Dave Young <dyoung@redhat.com>
> > > Cc: Hari Bathini <hbathini@linux.ibm.com>
> > > Cc: Jiri Bohac <jbohac@suse.cz>
> > > Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> > > Cc: Mahesh J Salgaonkar <mahesh@linux.ibm.com>
> > > Cc: Pingfan Liu <piliu@redhat.com>
> > > Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> > > Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
> > > Cc: Vivek Goyal <vgoyal@redhat.com>
> > > Cc: linuxppc-dev@lists.ozlabs.org
> > > Cc: kexec@lists.infradead.org
> > > Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> > > ---
> > > Changelog:
> > >   - Add the missing hunk to export crash_cma_ranges sysfs
> > > 
> > > ---
> > >   .../ABI/testing/sysfs-kernel-kexec-kdump        | 10 ++++++++++
> > >   kernel/kexec_core.c                             | 17 +++++++++++++++++
> > >   2 files changed, 27 insertions(+)
> > > 
> > > diff --git a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
> > > index 00c00f380fea..f59051b5d96d 100644
> > > --- a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
> > > +++ b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
> > > @@ -49,3 +49,13 @@ Description:	read only
> > >   		is used by the user space utility kexec to support updating the
> > >   		in-kernel kdump image during hotplug operations.
> > >   User:		Kexec tools
> > > +
> > > +What:		/sys/kernel/kexec/crash_cma_ranges
> > > +Date:		Nov 2025
> > > +Contact:	kexec@lists.infradead.org
> > > +Description:	read only
> > > +		Provides information about the memory ranges reserved from
> > > +		the Contiguous Memory Allocator (CMA) area that are allocated
> > > +		to the crash (kdump) kernel. It lists the start and end physical
> > > +		addresses of CMA regions assigned for crashkernel use.
> > > +User:		kdump service
> > > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > > index 7476a46de5d6..da6ff72b4669 100644
> > > --- a/kernel/kexec_core.c
> > > +++ b/kernel/kexec_core.c
> > > @@ -1271,6 +1271,22 @@ static ssize_t crash_size_store(struct kobject *kobj,
> > >   }
> > >   static struct kobj_attribute crash_size_attr = __ATTR_RW(crash_size);
> > > +static ssize_t crash_cma_ranges_show(struct kobject *kobj,
> > > +				     struct kobj_attribute *attr, char *buf)
> > > +{
> > > +
> > > +	ssize_t len = 0;
> > > +	int i;
> > > +
> > > +	for (i = 0; i < crashk_cma_cnt; ++i) {
> > > +		len += sysfs_emit_at(buf, len, "%08llx-%08llx\n",
> > > +				     crashk_cma_ranges[i].start,
> > > +				     crashk_cma_ranges[i].end);
> > > +	}
> > > +	return len;
> > > +}
> > > +static struct kobj_attribute crash_cma_ranges_attr = __ATTR_RO(crash_cma_ranges);
> > > +
> > >   #ifdef CONFIG_CRASH_HOTPLUG
> > >   static ssize_t crash_elfcorehdr_size_show(struct kobject *kobj,
> > >   			       struct kobj_attribute *attr, char *buf)
> > > @@ -1289,6 +1305,7 @@ static struct attribute *kexec_attrs[] = {
> > >   #ifdef CONFIG_CRASH_DUMP
> > >   	&crash_loaded_attr.attr,
> > >   	&crash_size_attr.attr,
> > > +	&crash_cma_ranges_attr.attr,
> > >   #ifdef CONFIG_CRASH_HOTPLUG
> > >   	&crash_elfcorehdr_size_attr.attr,
> > >   #endif
> > > -- 
> > > 2.51.1
> > > 
>