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

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Date: Sat, 30 Dec 2023 11:54:02 +0200
From: Nadav Amit <nadav.amit@...adcom.com>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Will Deacon <will@...nel.org>,
 "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 <linux-mm@...ck.org>,
 linux-arm-kernel@...ts.infradead.org,
 Linux Kernel Mailing List <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>,
 the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH 1/2] mm/tlb: fix fullmm semantics

> On Dec 28, 2023, at 10:46 AM, Jisheng Zhang <jszhang@...nel.org> 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;
> 

Thanks for pulling my patch out of the abyss, but the chunk above
did not come from my old patch.

My knowledge of arm64 is a bit limited, but the code does not seem
to match the comment, so if it is correct (which I strongly doubt),
the comment should be updated.

[1] https://lore.kernel.org/all/20210131001132.3368247-2-namit@vmware.com/

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