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

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Message-ID: <56abcc3f-ddd4-49c3-a985-a16d616e4210@linux.ibm.com>
Date: Tue, 11 Nov 2025 11:22:13 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: Baoquan he <bhe@...hat.com>
Cc: linux-kernel@...r.kernel.org, Aditya Gupta <adityag@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>, Hari Bathini <hbathini@...ux.ibm.com>,
        Jiri Bohac <jbohac@...e.cz>, Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Pingfan Liu <piliu@...hat.com>,
        "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
        Shivang Upadhyay <shivangu@...ux.ibm.com>,
        Vivek Goyal <vgoyal@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
        kexec@...ts.infradead.org
Subject: Re: [PATCH v3 5/5] crash: export crashkernel CMA reservation to
 userspace



On 11/11/25 06:45, Baoquan he wrote:
> 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.


Yeah, I will include it in v3. Thanks for the review.

- Sourabh Jain


>
>>>> 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@...ux.ibm.com>
>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>> Cc: Baoquan he <bhe@...hat.com>
>>>> Cc: Dave Young <dyoung@...hat.com>
>>>> Cc: Hari Bathini <hbathini@...ux.ibm.com>
>>>> Cc: Jiri Bohac <jbohac@...e.cz>
>>>> Cc: Madhavan Srinivasan <maddy@...ux.ibm.com>
>>>> Cc: Mahesh J Salgaonkar <mahesh@...ux.ibm.com>
>>>> Cc: Pingfan Liu <piliu@...hat.com>
>>>> Cc: Ritesh Harjani (IBM) <ritesh.list@...il.com>
>>>> Cc: Shivang Upadhyay <shivangu@...ux.ibm.com>
>>>> Cc: Vivek Goyal <vgoyal@...hat.com>
>>>> Cc: linuxppc-dev@...ts.ozlabs.org
>>>> Cc: kexec@...ts.infradead.org
>>>> Signed-off-by: Sourabh Jain <sourabhjain@...ux.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@...ts.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
>>>>