linux-kernel - [PATCH 1/3] x86/mm/pat: Disable preemption around __flush_tlb

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Message-Id: <20181016202525.29437-2-bigeasy@linutronix.de>
Date:   Tue, 16 Oct 2018 22:25:23 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 1/3] x86/mm/pat: Disable preemption around __flush_tlb_all()

I've seen the WARN_ON_ONCE(__read_cr3() != build_cr3()) in
switch_mm_irqs_off() every once in a while during a snapshotted system
upgrade.
I also saw the warning early during which was introduced in commit
decab0888e6e ("x86/mm: Remove preempt_disable/enable() from
__native_flush_tlb()"). The callchain is

  get_page_from_freelist() -> post_alloc_hook() -> __kernel_map_pages()

with CONFIG_DEBUG_PAGEALLOC enabled.

Turns out, once I disable preemption around __flush_tlb_all() both
warnings do not appear.

Disable preemption during CR3 reset / __flush_tlb_all().

Fixes: decab0888e6e ("x86/mm: Remove preempt_disable/enable() from __native_flush_tlb()")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/mm/pageattr.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 51a5a69ecac9f..fe6b21f0a6631 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -2088,7 +2088,9 @@ void __kernel_map_pages(struct page *page, int numpages, int enable)
 	 * We should perform an IPI and flush all tlbs,
 	 * but that can deadlock->flush only current cpu:
 	 */
+	preempt_disable();
 	__flush_tlb_all();
+	preempt_enable();
 
 	arch_flush_lazy_mmu_mode();
 }
-- 
2.19.1