linux-kernel - [tip:x86/urgent] x86/mm/hotplug: Fix BUG_ON() after hot-remove by not freeing PUD

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Date:   Tue, 27 Jun 2017 00:19:10 -0700
From:   "tip-bot for Jérôme Glisse" <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     luto@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com, tglx@...utronix.de,
        jglisse@...hat.com, hpa@...or.com, akpm@...ux-foundation.org,
        peterz@...radead.org, logang@...tatee.com,
        torvalds@...ux-foundation.org
Subject: [tip:x86/urgent] x86/mm/hotplug: Fix BUG_ON() after hot-remove by
 not freeing PUD

Commit-ID:  98fe3633c5a44e5ee3d642907739eb0210407886
Gitweb:     http://git.kernel.org/tip/98fe3633c5a44e5ee3d642907739eb0210407886
Author:     Jérôme Glisse <jglisse@...hat.com>
AuthorDate: Sat, 24 Jun 2017 14:05:14 -0400
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 26 Jun 2017 11:44:19 +0200

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 <jglisse@...hat.com>
[ Clarified the changelog and the code comments. ]
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Logan Gunthorpe <logang@...tatee.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-mm@...ck.org
Link: http://lkml.kernel.org/r/20170624180514.3821-1-jglisse@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 95651dc..0a59daf 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)