[RFC PATCH] x86_64/mm/boot: Fix kernel_ident_mapping_init() failure for kexec

[Xunlei Pang]

I found that the kdump is broken on linux-4.11.0-rc2+, probably
due to the 5level-paging feature that "#define p4d_present(p4d) 1",
as a result in ident_p4d_init(), it will go into ident_pud_init()
directly without allocating the new pud.

Looks like this patch can make it work again.

Cc: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Signed-off-by: Xunlei Pang <xlpang at redhat.com>
---
 arch/x86/mm/ident_map.c            | 22 ++++++++++------------
 include/asm-generic/5level-fixup.h |  6 +++---
 2 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/arch/x86/mm/ident_map.c b/arch/x86/mm/ident_map.c
index 04210a2..d14bb52 100644
--- a/arch/x86/mm/ident_map.c
+++ b/arch/x86/mm/ident_map.c
@@ -97,22 +97,20 @@ int kernel_ident_mapping_init(struct x86_mapping_info *info, pgd_t *pgd_page,
 			continue;
 		}
 
-		p4d = (p4d_t *)info->alloc_pgt_page(info->context);
-		if (!p4d)
-			return -ENOMEM;
+		if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
+			p4d = (p4d_t *)info->alloc_pgt_page(info->context);
+			if (!p4d)
+				return -ENOMEM;
+		} else {
+			p4d = pgd;
+		}
+
 		result = ident_p4d_init(info, p4d, addr, next);
 		if (result)
 			return result;
-		if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
+
+		if (IS_ENABLED(CONFIG_X86_5LEVEL))
 			set_pgd(pgd, __pgd(__pa(p4d) | _KERNPG_TABLE));
-		} else {
-			/*
-			 * With p4d folded, pgd is equal to p4d.
-			 * The pgd entry has to point to the pud page table in this case.
-			 */
-			pud_t *pud = pud_offset(p4d, 0);
-			set_pgd(pgd, __pgd(__pa(pud) | _KERNPG_TABLE));
-		}
 	}
 
 	return 0;
diff --git a/include/asm-generic/5level-fixup.h b/include/asm-generic/5level-fixup.h
index b5ca82d..3131ada 100644
--- a/include/asm-generic/5level-fixup.h
+++ b/include/asm-generic/5level-fixup.h
@@ -17,9 +17,9 @@
 
 #define p4d_alloc(mm, pgd, address)	(pgd)
 #define p4d_offset(pgd, start)		(pgd)
-#define p4d_none(p4d)			0
-#define p4d_bad(p4d)			0
-#define p4d_present(p4d)		1
+#define p4d_none(p4d)			pgd_none(p4d)
+#define p4d_bad(p4d)			pgd_bad(p4d)
+#define p4d_present(p4d)		pgd_present(p4d)
 #define p4d_ERROR(p4d)			do { } while (0)
 #define p4d_clear(p4d)			pgd_clear(p4d)
 #define p4d_val(p4d)			pgd_val(p4d)
-- 
1.8.3.1