[PATCH v2] kdump: round up the total memory size to 128M for crashkernel reservation

[Tao Liu]

The total memory size we get in kernel is usually slightly less than
the actual memory size because BIOS/firmware will reserve some memory
region. So it won't export all memory as usable.

E.g, on my x86_64 kvm guest with 1G memory, the total_mem value shows:
UEFI boot with ovmf:   0x3faef000
Legacy boot kvm guest: 0x3ff7ec00

When specifying crashkernel=1G-2G:128M, if we have a 1G memory machine,
we get total size 1023M from firmware. Then it will not fall into
1G-2G, thus no memory reserved. User will never know this, it is hard
to let user know the exact total value in kernel.

One way is to use dmi/smbios to get physical memory size, but it's not
reliable as well. According to Prarit hardware vendors sometimes screw
this up. Thus round up total size to 128M to work around this problem.

This patch is a resend of [1] and rebased onto v5.19-rc2, and the
original credit goes to Dave Young <dyoung@xxxxxxxxxx>.

[1]: http://lists.infradead.org/pipermail/kexec/2018-April/020568.html

Signed-off-by: Tao Liu <ltao@xxxxxxxxxx>
---
v1 -> v2:
Modified commit log based on Baoquan's advice.
---
 kernel/crash_core.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 71122e01623c..b58b27cbdb61 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -9,6 +9,7 @@
 #include <linux/init.h>
 #include <linux/utsname.h>
 #include <linux/vmalloc.h>
+#include <linux/sizes.h>
 
 #include <asm/page.h>
 #include <asm/sections.h>
@@ -43,6 +44,15 @@ static int __init parse_crashkernel_mem(char *cmdline,
 					unsigned long long *crash_base)
 {
 	char *cur = cmdline, *tmp;
+	unsigned long long total_mem = system_ram;
+
+	/*
+	 * Firmware sometimes reserves some memory regions for its own use,
+	 * so the system memory size is less than the actual physical memory
+	 * size. Work around this by rounding up the total size to 128M,
+	 * which is enough for most test cases.
+	 */
+	total_mem = roundup(total_mem, SZ_128M);
 
 	/* for each entry of the comma-separated list */
 	do {
@@ -87,13 +97,13 @@ static int __init parse_crashkernel_mem(char *cmdline,
 			return -EINVAL;
 		}
 		cur = tmp;
-		if (size >= system_ram) {
+		if (size >= total_mem) {
 			pr_warn("crashkernel: invalid size\n");
 			return -EINVAL;
 		}
 
 		/* match ? */
-		if (system_ram >= start && system_ram < end) {
+		if (total_mem >= start && total_mem < end) {
 			*crash_size = size;
 			break;
 		}
-- 
2.33.1