linux-kernel - Re: [PATCH 01/11] x86/mm: make MMU_GATHER_RCU_TABLE

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Message-ID: <20241224182642.GC17252@noisy.programming.kicks-ass.net>
Date: Tue, 24 Dec 2024 19:26:42 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Rik van Riel <riel@...riel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	dave.hansen@...ux.intel.com, luto@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, hpa@...or.com,
	akpm@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH 01/11] x86/mm: make MMU_GATHER_RCU_TABLE_FREE
 unconditional

On Sun, Dec 22, 2024 at 09:55:07PM -0500, Rik van Riel wrote:
> Currently x86 usse CONFIG_MMU_GATHER_TABLE_FREE when using
> paravirt, and not when running on bare metal.
> 
> There is no real good reason to do things differently for
> each setup. Make them all the same.

More importantly, the changes you're proposing very much rely on this.
Without TLBi IPIs nothing serializes GUP-fast vs TLBi and this RCU-ish
table free scheme is required.

> Signed-off-by: Rik van Riel <riel@...riel.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/x86/Kconfig           | 2 +-
>  arch/x86/kernel/paravirt.c | 7 +------
>  2 files changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 9d7bd0ae48c4..e8743f8c9fd0 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -274,7 +274,7 @@ config X86
>  	select HAVE_PCI
>  	select HAVE_PERF_REGS
>  	select HAVE_PERF_USER_STACK_DUMP
> -	select MMU_GATHER_RCU_TABLE_FREE	if PARAVIRT
> +	select MMU_GATHER_RCU_TABLE_FREE
>  	select MMU_GATHER_MERGE_VMAS
>  	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
>  	select HAVE_REGS_AND_STACK_ACCESS_API
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index fec381533555..2b78a6b466ed 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -59,11 +59,6 @@ void __init native_pv_lock_init(void)
>  		static_branch_enable(&virt_spin_lock_key);
>  }
>  
> -static void native_tlb_remove_table(struct mmu_gather *tlb, void *table)
> -{
> -	tlb_remove_page(tlb, table);
> -}
> -
>  struct static_key paravirt_steal_enabled;
>  struct static_key paravirt_steal_rq_enabled;
>  
> @@ -191,7 +186,7 @@ struct paravirt_patch_template pv_ops = {
>  	.mmu.flush_tlb_kernel	= native_flush_tlb_global,
>  	.mmu.flush_tlb_one_user	= native_flush_tlb_one_user,
>  	.mmu.flush_tlb_multi	= native_flush_tlb_multi,
> -	.mmu.tlb_remove_table	= native_tlb_remove_table,
> +	.mmu.tlb_remove_table	= tlb_remove_table,
>  
>  	.mmu.exit_mmap		= paravirt_nop,
>  	.mmu.notify_page_enc_status_changed	= paravirt_nop,
> -- 
> 2.47.1
>