[PATCH] crash: Support high memory reservation for range syntax

[Youling Tang <youling.tang@linux.dev>]

From: Youling Tang <tangyouling@kylinos.cn>

The crashkernel range syntax (range1:size1[,range2:size2,...]) allows
automatic size selection based on system RAM, but it always reserves
from low memory. When a large crashkernel is selected, this can
consume most of the low memory, causing subsequent hardware
hotplug or drivers requiring low memory to fail due to allocation
failures.

Add a new optional conditional suffix ",>boundary" to the crashkernel
range syntax. When the selected crashkernel size exceeds the specified
boundary, the kernel will automatically apply the same reservation
policy as "crashkernel=size,high" - preferring high memory first
and reserving the default low memory area.

Syntax:
    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset],>boundary

Example:
    crashkernel=2G-16G:512M,16G-:1G,>512M

This means:
  - For 2G-16G RAM: reserve 512M normally
  - For >16G RAM: reserve 1G with high memory preference (since 1G > 512M)

For systems with >16G RAM, 1G is selected which exceeds 512M, so it
will be reserved from high memory instead of consuming 1G of
precious low memory.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
---
 Documentation/admin-guide/kdump/kdump.rst     | 25 ++++++++-
 .../admin-guide/kernel-parameters.txt         |  2 +-
 kernel/crash_reserve.c                        | 56 ++++++++++++++++---
 3 files changed, 73 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 7587caadbae1..b5ae4556e9ca 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -293,7 +293,28 @@ crashkernel syntax
        2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
        3) if the RAM size is larger than 2G, then reserve 128M
 
-3) crashkernel=size,high and crashkernel=size,low
+3) range1:size1[,range2:size2,...][@offset],>boundary
+   Optionally, the range list can be followed by a conditional suffix
+   `,>boundary`. When the selected crashkernel size matches the
+   condition, the kernel will reserve memory using the same policy as
+   `crashkernel=size,high` (i.e. prefer high memory first and reserve the
+   default low memory area).
+
+   The syntax is::
+
+        crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset],>boundary
+        range=start-[end]
+
+   For example::
+
+        crashkernel=2G-16G:512M,16G-:1G,>512M
+
+   This would mean:
+       1) if the RAM size is between 2G and 16G (exclusive), then reserve 512M.
+       2) if the RAM size is larger than 16G, allocation will behave like
+          `crashkernel=1G,high`.
+
+4) crashkernel=size,high and crashkernel=size,low
 
    If memory above 4G is preferred, crashkernel=size,high can be used to
    fulfill that. With it, physical memory is allowed to be allocated from top,
@@ -311,7 +332,7 @@ crashkernel syntax
 
             crashkernel=0,low
 
-4) crashkernel=size,cma
+5) crashkernel=size,cma
 
 	Reserve additional crash kernel memory from CMA. This reservation is
 	usable by the first system's userspace memory and kernel movable
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 03a550630644..b2e1892ab4d8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1087,7 +1087,7 @@ Kernel parameters
 			4G when '@offset' hasn't been specified.
 			See Documentation/admin-guide/kdump/kdump.rst for further details.
 
-	crashkernel=range1:size1[,range2:size2,...][@offset]
+	crashkernel=range1:size1[,range2:size2,...][@offset][,>boundary]
 			[KNL] Same as above, but depends on the memory
 			in the running system. The syntax of range is
 			start-[end] where start and end are both
diff --git a/kernel/crash_reserve.c b/kernel/crash_reserve.c
index 62e60e0223cf..917738412390 100644
--- a/kernel/crash_reserve.c
+++ b/kernel/crash_reserve.c
@@ -254,15 +254,47 @@ static __init char *get_last_crashkernel(char *cmdline,
 	return ck_cmdline;
 }
 
+/*
+ * This function parses command lines in the format
+ *
+ *   crashkernel=ramsize-range:size[,...][@offset],>boundary
+ */
+static void __init parse_crashkernel_boundary(char *ck_cmdline,
+					unsigned long long *boundary)
+{
+	char *cur = ck_cmdline, *next;
+	char *first_gt = false;
+
+	first_gt = strchr(cur, '>');
+	if (!first_gt)
+		return;
+
+	cur = first_gt + 1;
+	if (*cur == '\0' || *cur == ' ' || *cur == ',') {
+		pr_warn("crashkernel: '>' specified without boundary size, ignoring\n");
+		return;
+	}
+
+	*boundary = memparse(cur, &next);
+	if (cur == next) {
+		pr_warn("crashkernel: invalid boundary size after '>'\n");
+		return;
+	}
+}
+
 static int __init __parse_crashkernel(char *cmdline,
 			     unsigned long long system_ram,
 			     unsigned long long *crash_size,
 			     unsigned long long *crash_base,
-			     const char *suffix)
+			     const char *suffix,
+			     bool *high,
+			     unsigned long long *low_size)
 {
 	char *first_colon, *first_space;
 	char *ck_cmdline;
 	char *name = "crashkernel=";
+	unsigned long long boundary = 0;
+	int ret;
 
 	BUG_ON(!crash_size || !crash_base);
 	*crash_size = 0;
@@ -283,10 +315,20 @@ static int __init __parse_crashkernel(char *cmdline,
 	 */
 	first_colon = strchr(ck_cmdline, ':');
 	first_space = strchr(ck_cmdline, ' ');
-	if (first_colon && (!first_space || first_colon < first_space))
-		return parse_crashkernel_mem(ck_cmdline, system_ram,
+	if (first_colon && (!first_space || first_colon < first_space)) {
+		ret = parse_crashkernel_mem(ck_cmdline, system_ram,
 				crash_size, crash_base);
 
+		/* Handle optional ',>boundary' condition for range ':' syntax only. */
+		parse_crashkernel_boundary(ck_cmdline, &boundary);
+		if (!ret && *crash_size > boundary) {
+			*high = true;
+			*low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
+		}
+
+		return ret;
+	}
+
 	return parse_crashkernel_simple(ck_cmdline, crash_size, crash_base);
 }
 
@@ -310,7 +352,7 @@ int __init parse_crashkernel(char *cmdline,
 
 	/* crashkernel=X[@offset] */
 	ret = __parse_crashkernel(cmdline, system_ram, crash_size,
-				crash_base, NULL);
+				crash_base, NULL, high, low_size);
 #ifdef CONFIG_ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
 	/*
 	 * If non-NULL 'high' passed in and no normal crashkernel
@@ -318,7 +360,7 @@ int __init parse_crashkernel(char *cmdline,
 	 */
 	if (high && ret == -ENOENT) {
 		ret = __parse_crashkernel(cmdline, 0, crash_size,
-				crash_base, suffix_tbl[SUFFIX_HIGH]);
+				crash_base, suffix_tbl[SUFFIX_HIGH], high, low_size);
 		if (ret || !*crash_size)
 			return -EINVAL;
 
@@ -327,7 +369,7 @@ int __init parse_crashkernel(char *cmdline,
 		 * is not allowed.
 		 */
 		ret = __parse_crashkernel(cmdline, 0, low_size,
-				crash_base, suffix_tbl[SUFFIX_LOW]);
+				crash_base, suffix_tbl[SUFFIX_LOW], high, low_size);
 		if (ret == -ENOENT) {
 			*low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
 			ret = 0;
@@ -344,7 +386,7 @@ int __init parse_crashkernel(char *cmdline,
 	 */
 	if (cma_size)
 		__parse_crashkernel(cmdline, 0, cma_size,
-			&cma_base, suffix_tbl[SUFFIX_CMA]);
+			&cma_base, suffix_tbl[SUFFIX_CMA], high, low_size);
 #endif
 	if (!*crash_size)
 		ret = -EINVAL;
-- 
2.43.0