x86/mm/hotplug: Fix BUG_ON() after hot-remove by not freeing PUD

Since commit:

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

we no longer free PUDs so that we do not have to synchronize
all PGDs on hot-remove/vfree().

But the new 5-level page table patchset reverted that for 4-level
page tables, in the following commit:

  f2a6a7050109: ("x86: Convert the rest of the code to support p4d_t")

This patch restores the damage and disables free_pud() if we are in the
4-level page table case, thus avoiding BUG_ON() after hot-remove.

Signed-off-by: Jérôme Glisse <[email protected]>
[ Clarified the changelog and the code comments. ]
Reviewed-by: Kirill A. Shutemov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 95651dc58e09..0a59daf799f8 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-level page tables we do not want to free PUDs, but in the
+		 * 5-level case we should free them. This code will have to change
+		 * to adapt for boot-time switching between 4 and 5 level page tables.
+		 */
+		if (CONFIG_PGTABLE_LEVELS == 5)
+			free_pud_table(pud_base, p4d);
 	}
 
 	if (direct)