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Date:   Tue, 4 Apr 2023 17:12:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yair Podemsky <ypodemsk@redhat.com>
Cc:     linux@armlinux.org.uk, mpe@ellerman.id.au, npiggin@gmail.com,
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        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode
Message-ID: <20230404151217.GB297936@hirez.programming.kicks-ass.net>
References: <20230404134224.137038-1-ypodemsk@redhat.com>
 <20230404134224.137038-4-ypodemsk@redhat.com>
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On Tue, Apr 04, 2023 at 04:42:24PM +0300, Yair Podemsky wrote:
> The tlb_remove_table_smp_sync IPI is used to ensure the outdated tlb page
> is not currently being accessed and can be cleared.
> This occurs once all CPUs have left the lockless gup code section.
> If they reenter the page table walk, the pointers will be to the new
> pages.
> Therefore the IPI is only needed for CPUs in kernel mode.
> By preventing the IPI from being sent to CPUs not in kernel mode,
> Latencies are reduced.
> 
> Race conditions considerations:
> The context state check is vulnerable to race conditions between the
> moment the context state is read to when the IPI is sent (or not).
> 
> Here are these scenarios.
> case 1:
> CPU-A                                             CPU-B
> 
>                                                   state == CONTEXT_KERNEL
> int state = atomic_read(&ct->state);
>                                                   Kernel-exit:
>                                                   state == CONTEXT_USER
> if (state & CT_STATE_MASK == CONTEXT_KERNEL)
> 
> In this case, the IPI will be sent to CPU-B despite it is no longer in
> the kernel. The consequence of which would be an unnecessary IPI being
> handled by CPU-B, causing a reduction in latency.
> This would have been the case every time without this patch.
> 
> case 2:
> CPU-A                                             CPU-B
> 
> modify pagetables
> tlb_flush (memory barrier)
>                                                   state == CONTEXT_USER
> int state = atomic_read(&ct->state);
>                                                   Kernel-enter:
>                                                   state == CONTEXT_KERNEL
>                                                   READ(pagetable values)
> if (state & CT_STATE_MASK == CONTEXT_USER)
> 
> In this case, the IPI will not be sent to CPU-B despite it returning to
> the kernel and even reading the pagetable.
> However since this CPU-B has entered the pagetable after the
> modification it is reading the new, safe values.
> 
> The only case when this IPI is truly necessary is when CPU-B has entered
> the lockless gup code section before the pagetable modifications and
> has yet to exit them, in which case it is still in the kernel.
> 
> Signed-off-by: Yair Podemsky <ypodemsk@redhat.com>
> ---
>  mm/mmu_gather.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index 5ea9be6fb87c..731d955e152d 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -9,6 +9,7 @@
>  #include <linux/smp.h>
>  #include <linux/swap.h>
>  #include <linux/rmap.h>
> +#include <linux/context_tracking_state.h>
>  
>  #include <asm/pgalloc.h>
>  #include <asm/tlb.h>
> @@ -191,6 +192,20 @@ static void tlb_remove_table_smp_sync(void *arg)
>  	/* Simply deliver the interrupt */
>  }
>  
> +
> +#ifdef CONFIG_CONTEXT_TRACKING
> +static bool cpu_in_kernel(int cpu, void *info)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> +	int state = atomic_read(&ct->state);
> +	/* will return true only for cpus in kernel space */
> +	return state & CT_STATE_MASK == CONTEXT_KERNEL;
> +}
> +#define CONTEXT_PREDICATE cpu_in_kernel
> +#else
> +#define CONTEXT_PREDICATE NULL
> +#endif /* CONFIG_CONTEXT_TRACKING */
> +
>  #ifdef CONFIG_ARCH_HAS_CPUMASK_BITS
>  #define REMOVE_TABLE_IPI_MASK mm_cpumask(mm)
>  #else
> @@ -206,8 +221,8 @@ void tlb_remove_table_sync_one(struct mm_struct *mm)
>  	 * It is however sufficient for software page-table walkers that rely on
>  	 * IRQ disabling.
>  	 */
> -	on_each_cpu_mask(REMOVE_TABLE_IPI_MASK, tlb_remove_table_smp_sync,
> -			NULL, true);
> +	on_each_cpu_cond_mask(CONTEXT_PREDICATE, tlb_remove_table_smp_sync,
> +			NULL, true, REMOVE_TABLE_IPI_MASK);
>  }

I think this is correct; but... I would like much of the changelog
included in a comment above cpu_in_kernel(). I'm sure someone will try
and read this code and wonder about those race conditions.

Of crucial importance is the fact that the page-table modification comes
before the tlbi.

Also, do we really not already have this helper function somewhere, it
seems like something obvious to already have, Frederic?