linux-kernel - Re: [PATCH v1 2/4] sparc/mm: Disable preemption in lazy mmu mode

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Message-ID: <4af46304-cf25-4c4a-8e4a-3a566193ca62@redhat.com>
Date: Mon, 3 Mar 2025 09:51:34 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "David S. Miller" <davem@...emloft.net>,
 Andreas Larsson <andreas@...sler.com>, Juergen Gross <jgross@...e.com>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Catalin Marinas <catalin.marinas@....com>
Cc: linux-mm@...ck.org, sparclinux@...r.kernel.org,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/4] sparc/mm: Disable preemption in lazy mmu mode

On 02.03.25 15:55, Ryan Roberts wrote:
> Since commit 38e0edb15bd0 ("mm/apply_to_range: call pte function with
> lazy updates") it's been possible for arch_[enter|leave]_lazy_mmu_mode()
> to be called without holding a page table lock (for the kernel mappings
> case), and therefore it is possible that preemption may occur while in
> the lazy mmu mode. The Sparc lazy mmu implementation is not robust to
> preemption since it stores the lazy mode state in a per-cpu structure
> and does not attempt to manage that state on task switch.
> 
> Powerpc had the same issue and fixed it by explicitly disabling
> preemption in arch_enter_lazy_mmu_mode() and re-enabling in
> arch_leave_lazy_mmu_mode(). See commit b9ef323ea168 ("powerpc/64s:
> Disable preemption in hash lazy mmu mode").
> 
> Given Sparc's lazy mmu mode is based on powerpc's, let's fix it in the
> same way here.
> 
> Fixes: 38e0edb15bd0 ("mm/apply_to_range: call pte function with lazy updates")
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>   arch/sparc/mm/tlb.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/sparc/mm/tlb.c b/arch/sparc/mm/tlb.c
> index 8648a50afe88..a35ddcca5e76 100644
> --- a/arch/sparc/mm/tlb.c
> +++ b/arch/sparc/mm/tlb.c
> @@ -52,8 +52,10 @@ void flush_tlb_pending(void)
>   
>   void arch_enter_lazy_mmu_mode(void)
>   {
> -	struct tlb_batch *tb = this_cpu_ptr(&tlb_batch);
> +	struct tlb_batch *tb;
>   
> +	preempt_disable();
> +	tb = this_cpu_ptr(&tlb_batch);
>   	tb->active = 1;
>   }
>   
> @@ -64,6 +66,7 @@ void arch_leave_lazy_mmu_mode(void)
>   	if (tb->tlb_nr)
>   		flush_tlb_pending();
>   	tb->active = 0;
> +	preempt_enable();
>   }
>   
>   static void tlb_batch_add_one(struct mm_struct *mm, unsigned long vaddr,

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb