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

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Date:   Wed, 17 Oct 2018 09:54:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sebastian Andrzej Siewior' <bigeasy@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "x86@...nel.org" <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>
Subject: RE: [PATCH 1/3] x86/mm/pat: Disable preemption around
 __flush_tlb_all()

From: Sebastian Andrzej Siewior
> Sent: 16 October 2018 21:25
> 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();

Can it make any sense to flush the tlb with preemption enabled?
Surely preemption must be disabled over something else as well?

	David

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