linux-kernel - Re: [PATCH v3 04/11] x86/mm: Give each mm TLB flush generation a unique ID

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Date:   Wed, 21 Jun 2017 12:33:22 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 04/11] x86/mm: Give each mm TLB flush generation a
 unique ID

On Tue, Jun 20, 2017 at 10:22:10PM -0700, Andy Lutomirski wrote:
> - * The x86 doesn't have a mmu context, but
> - * we put the segment information here.
> + * x86 has arch-specific MMU state beyond what lives in mm_struct.
>   */
>  typedef struct {
> +	/*
> +	 * ctx_id uniquely identifies this mm_struct.  A ctx_id will never
> +	 * be reused, and zero is not a valid ctx_id.
> +	 */
> +	u64 ctx_id;
> +
> +	/*
> +	 * Any code that needs to do any sort of TLB flushing for this
> +	 * mm will first make its changes to the page tables, then
> +	 * increment tlb_gen, then flush.  This lets the low-level
> +	 * flushing code keep track of what needs flushing.
> +	 *
> +	 * This is not used on Xen PV.
> +	 */
> +	atomic64_t tlb_gen;
> +
>  #ifdef CONFIG_MODIFY_LDT_SYSCALL
>  	struct ldt_struct *ldt;
>  #endif
> @@ -37,6 +53,11 @@ typedef struct {
>  #endif
>  } mm_context_t;
>  
> +#define INIT_MM_CONTEXT(mm)						\
> +	.context = {							\
> +		.ctx_id = 1,						\

So ctx_id of 0 is invalid?

Let's state that explicitly. We could even use it to sanity-check mms or
whatever.

> +	}
> +
>  void leave_mm(int cpu);
>  
>  #endif /* _ASM_X86_MMU_H */
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index ecfcb6643c9b..e5295d485899 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -129,9 +129,14 @@ static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
>  		this_cpu_write(cpu_tlbstate.state, TLBSTATE_LAZY);
>  }
>  
> +extern atomic64_t last_mm_ctx_id;

I think we prefer externs/variable defines at the beginning of the file,
not intermixed with functions.

> +
>  static inline int init_new_context(struct task_struct *tsk,
>  				   struct mm_struct *mm)
>  {
> +	mm->context.ctx_id = atomic64_inc_return(&last_mm_ctx_id);
> +	atomic64_set(&mm->context.tlb_gen, 0);
> +
>  	#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>  	if (cpu_feature_enabled(X86_FEATURE_OSPKE)) {
>  		/* pkey 0 is the default and always allocated */
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index 50ea3482e1d1..1eb946c0507e 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -57,6 +57,23 @@ static inline void invpcid_flush_all_nonglobals(void)
>  	__invpcid(0, 0, INVPCID_TYPE_ALL_NON_GLOBAL);
>  }
>  
> +static inline u64 bump_mm_tlb_gen(struct mm_struct *mm)

inc_mm_tlb_gen() I guess. git grep says like "inc" more :-)

> +{
> +	u64 new_tlb_gen;
> +
> +	/*
> +	 * Bump the generation count.  This also serves as a full barrier
> +	 * that synchronizes with switch_mm: callers are required to order

Please end function names with parentheses.

> +	 * their read of mm_cpumask after their writes to the paging
> +	 * structures.
> +	 */
> +	smp_mb__before_atomic();
> +	new_tlb_gen = atomic64_inc_return(&mm->context.tlb_gen);
> +	smp_mb__after_atomic();
> +
> +	return new_tlb_gen;
> +}
> +
>  #ifdef CONFIG_PARAVIRT
>  #include <asm/paravirt.h>
>  #else

-- 
Regards/Gruss,
    Boris.

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