linux-kernel - Re: [PATCH 1/2] mm/tlb: fix fullmm semantics

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Message-ID: <20240103175001.GF5954@willie-the-truck>
Date: Wed, 3 Jan 2024 17:50:01 +0000
From: Will Deacon <will@...nel.org>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org, Nadav Amit <namit@...are.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>, Yu Zhao <yuzhao@...gle.com>,
	x86@...nel.org
Subject: Re: [PATCH 1/2] mm/tlb: fix fullmm semantics

On Thu, Dec 28, 2023 at 04:46:41PM +0800, Jisheng Zhang wrote:
> From: Nadav Amit <namit@...are.com>
> 
> fullmm in mmu_gather is supposed to indicate that the mm is torn-down
> (e.g., on process exit) and can therefore allow certain optimizations.
> However, tlb_finish_mmu() sets fullmm, when in fact it want to say that
> the TLB should be fully flushed.
> 
> Change tlb_finish_mmu() to set need_flush_all and check this flag in
> tlb_flush_mmu_tlbonly() when deciding whether a flush is needed.
> 
> At the same time, bring the arm64 fullmm on process exit optimization back.
> 
> Signed-off-by: Nadav Amit <namit@...are.com>
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Will Deacon <will@...nel.org>
> Cc: Yu Zhao <yuzhao@...gle.com>
> Cc: Nick Piggin <npiggin@...il.com>
> Cc: x86@...nel.org
> ---
>  arch/arm64/include/asm/tlb.h | 5 ++++-
>  include/asm-generic/tlb.h    | 2 +-
>  mm/mmu_gather.c              | 2 +-
>  3 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
> index 846c563689a8..6164c5f3b78f 100644
> --- a/arch/arm64/include/asm/tlb.h
> +++ b/arch/arm64/include/asm/tlb.h
> @@ -62,7 +62,10 @@ static inline void tlb_flush(struct mmu_gather *tlb)
>  	 * invalidating the walk-cache, since the ASID allocator won't
>  	 * reallocate our ASID without invalidating the entire TLB.
>  	 */
> -	if (tlb->fullmm) {
> +	if (tlb->fullmm)
> +		return;
> +
> +	if (tlb->need_flush_all) {
>  		if (!last_level)
>  			flush_tlb_mm(tlb->mm);
>  		return;

Why isn't the 'last_level' check sufficient here? In other words, when do
we perform a !last_level invalidation with 'fullmm' set outside of teardown?

Will