linux-kernel - [PATCH v3 6/7] powerpc: insert System RAM resource to prevent crashkernel conflict

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Message-ID: <20250131113830.925179-7-sourabhjain@linux.ibm.com>
Date: Fri, 31 Jan 2025 17:08:29 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: linuxppc-dev@...ts.ozlabs.org
Cc: Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>, Baoquan he <bhe@...hat.com>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 6/7] powerpc: insert System RAM resource to prevent crashkernel conflict

The next patch in the series with title "powerpc/crash: use generic
crashkernel reservation" enables powerpc to use generic crashkernel
reservation instead of custom implementation. This leads to exporting
of `Crash Kernel` memory to iomem_resource (/proc/iomem) via
insert_crashkernel_resources():kernel/crash_reserve.c or at another
place in the same file if
HAVE_ARCH_ADD_CRASH_RES_TO_IOMEM_EARLY is set.

The add_system_ram_resources():arch/powerpc/mm/mem.c adds `System RAM`
to iomem_resource using request_resource(). This creates a conflict with
`Crash Kernel`, which is added by the generic crashkernel reservation
code. As a result, the kernel ultimately fails to add `System RAM` to
iomem_resource. Consequently, it does not appear in /proc/iomem.

There are multiple approches tried to avoid this:

1. Don't add Crash Kernel to iomem_resource:
    - This has two issues.
      First, it requires adding an architecture-specific hook in the
      generic code. There are already two code paths to choose when to
      add `Crash Kernel` to iomem_resource. This adds one more code path
      to skip it.

      Second, what if `Crash Kernel` is required in /proc/iomem in the
      future? Many architectures do export it.

2. Don't add `System RAM` to iomem_resource by reverting commit
   c40dd2f766440 ("powerpc: Add System RAM to /proc/iomem"):
    - It's not ideal to export `System RAM` via /proc/iomem, but since
      it already done ealier and userspace tools like kdump and
      kdump-utils rely on `System RAM` from /proc/iomem, removing it
      will break userspace.

3. Add Crash Kernel along with System RAM to /proc/iomem:

This patch takes the third approach by updating
add_system_ram_resources() to use insert_resource() instead of the
request_resource() API to add the `System RAM` resource to
iomem_resource. insert_resource() allows inserting resources even if
they overlap with existing ones. Since `Crash Kernel` and `System RAM`
resources are added to iomem_resource early in the boot, any other
conflict is not expected.

With the changes introduced here and in the next patch, "powerpc/crash:
use generic crashkernel reservation," /proc/iomem now exports
`System RAM` and `Crash Kernel` as shown below:

$ cat /proc/iomem
00000000-3ffffffff : System RAM
  10000000-4fffffff : Crash kernel

The kdump script is capable of handling `System RAM` and `Crash Kernel`
in the above format. The same format is used in other architectures.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Baoquan he <bhe@...hat.com>
Cc: Hari Bathini <hbathini@...ux.ibm.com>
CC: Madhavan Srinivasan <maddy@...ux.ibm.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: kexec@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Sourabh Jain <sourabhjain@...ux.ibm.com>
---
 arch/powerpc/mm/mem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index c7708c8fad29..615966d71425 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -376,7 +376,7 @@ static int __init add_system_ram_resources(void)
 			 */
 			res->end = end - 1;
 			res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
-			WARN_ON(request_resource(&iomem_resource, res) < 0);
+			WARN_ON(insert_resource(&iomem_resource, res) < 0);
 		}
 	}

-- 
2.48.1