[PATCH] x86/mm/hotplug: fix BUG_ON() after hotremove by not freeing pud v3

[jglisse]

From: JÃrÃme Glisse <jglisse@xxxxxxxxxx>

With commit af2cf278ef4f we no longer free pud so that we do not
have synchronize all pgd on hotremove/vfree. But the new 5 level
page table patchset reverted that for 4 level page table.

This patch restore af2cf278ef4f and disable free_pud() if we are
in the 4 level page table case thus avoiding BUG_ON() after hot-
remove.

af2cf278ef4f x86/mm/hotplug: Don't remove PGD entries in remove_pagetable()

Changed since v2:
  - nove to if the callsite instead of having special version of
    free_pud for 4 level page table
Changed since v1:
  - make free_pud() conditional on the number of page table
    level
  - improved commit message

Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Reviwed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 arch/x86/mm/init_64.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 95651dc58e09..dc4c99f9ca58 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -990,7 +990,13 @@ remove_p4d_table(p4d_t *p4d_start, unsigned long addr, unsigned long end,
 
 		pud_base = pud_offset(p4d, 0);
 		remove_pud_table(pud_base, addr, next, direct);
-		free_pud_table(pud_base, p4d);
+		/*
+		 * For 4 levels page table we do not want to free puds but for
+		 * 5 levels we should free them. This code also need to change
+		 * to adapt for boot time switching between 4 and 5 level.
+		 */
+		if (CONFIG_PGTABLE_LEVELS == 5)
+			free_pud_table(pud_base, p4d);
 	}
 
 	if (direct)
-- 
2.13.0