powerpc/fadump: Do not use hugepages when fadump is active

FADump capture kernel boots in restricted memory environment preserving
the context of previous kernel to save vmcore. Supporting hugepages in
such environment makes things unnecessarily complicated, as hugepages
need memory set aside for them. This means most of the capture kernel's
memory is used in supporting hugepages. In most cases, this results in
out-of-memory issues while booting FADump capture kernel. But hugepages
are not of much use in capture kernel whose only job is to save vmcore.
So, disabling hugepages support, when fadump is active, is a reliable
solution for the out of memory issues. Introducing a flag variable to
disable HugeTLB support when fadump is active.
Signed-off-by: Hari Bathini <hbathini@linux.vnet.ibm.com>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/page.h

@@ -39,6 +39,7 @@
 
 #ifndef __ASSEMBLY__
 #ifdef CONFIG_HUGETLB_PAGE
+extern bool hugetlb_disabled;
 extern unsigned int HPAGE_SHIFT;
 #else
 #define HPAGE_SHIFT PAGE_SHIFT

arch/powerpc/kernel/fadump.c

@@ -402,6 +402,14 @@ int __init fadump_reserve_mem(void)
 	if (fw_dump.dump_active) {
 		pr_info("Firmware-assisted dump is active.\n");
 
+#ifdef CONFIG_HUGETLB_PAGE
+		/*
+		 * FADump capture kernel doesn't care much about hugepages.
+		 * In fact, handling hugepages in capture kernel is asking for
+		 * trouble. So, disable HugeTLB support when fadump is active.
+		 */
+		hugetlb_disabled = true;
+#endif
 		/*
 		 * If last boot has crashed then reserve all the memory
 		 * above boot_memory_size so that we don't touch it until

arch/powerpc/mm/hash_utils_64.c

@@ -572,8 +572,10 @@ static void __init htab_scan_page_sizes(void)
 	}
 
 #ifdef CONFIG_HUGETLB_PAGE
+	if (!hugetlb_disabled) {
 		/* Reserve 16G huge page memory sections for huge pages */
 		of_scan_flat_dt(htab_dt_scan_hugepage_blocks, NULL);
+	}
 #endif /* CONFIG_HUGETLB_PAGE */
 }
 

arch/powerpc/mm/hugetlbpage.c

@@ -35,6 +35,8 @@
 #define PAGE_SHIFT_16M	24
 #define PAGE_SHIFT_16G	34
 
+bool hugetlb_disabled = false;
+
 unsigned int HPAGE_SHIFT;
 EXPORT_SYMBOL(HPAGE_SHIFT);
 
@@ -651,6 +653,11 @@ static int __init hugetlbpage_init(void)
 {
 	int psize;
 
+	if (hugetlb_disabled) {
+		pr_info("HugeTLB support is disabled!\n");
+		return 0;
+	}
+
 #if !defined(CONFIG_PPC_FSL_BOOK3E) && !defined(CONFIG_PPC_8xx)
 	if (!radix_enabled() && !mmu_has_feature(MMU_FTR_16M_PAGE))
 		return -ENODEV;