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Subject: Re: [PATCH 1/2] mm/tlb: fix fullmm semantics
From: Nadav Amit <nadav.amit@broadcom.com>
In-Reply-To: <20231228084642.1765-2-jszhang@kernel.org>
Date: Sat, 30 Dec 2023 11:54:02 +0200
Cc: Will Deacon <will@kernel.org>,
 "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Nick Piggin <npiggin@gmail.com>,
 Peter Zijlstra <peterz@infradead.org>,
 Catalin Marinas <catalin.marinas@arm.com>,
 Paul Walmsley <paul.walmsley@sifive.com>,
 Palmer Dabbelt <palmer@dabbelt.com>,
 Albert Ou <aou@eecs.berkeley.edu>,
 Arnd Bergmann <arnd@arndb.de>,
 linux-arch@vger.kernel.org,
 linux-mm <linux-mm@kvack.org>,
 linux-arm-kernel@lists.infradead.org,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
 linux-riscv@lists.infradead.org,
 Nadav Amit <namit@vmware.com>,
 Andrea Arcangeli <aarcange@redhat.com>,
 Andy Lutomirski <luto@kernel.org>,
 Dave Hansen <dave.hansen@linux.intel.com>,
 Thomas Gleixner <tglx@linutronix.de>,
 Yu Zhao <yuzhao@google.com>,
 the arch/x86 maintainers <x86@kernel.org>
Message-Id: <204B6410-2EFA-462B-9DF7-64CC5F1D3AD2@broadcom.com>
References: <20231228084642.1765-1-jszhang@kernel.org>
 <20231228084642.1765-2-jszhang@kernel.org>
To: Jisheng Zhang <jszhang@kernel.org>
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> On Dec 28, 2023, at 10:46=E2=80=AFAM, Jisheng Zhang <jszhang@kernel.org> =
wrote:
>=20
> From: Nadav Amit <namit@vmware.com>
>=20
> 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.
>=20
> 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.
>=20
> At the same time, bring the arm64 fullmm on process exit optimization bac=
k.
>=20
> Signed-off-by: Nadav Amit <namit@vmware.com>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Nick Piggin <npiggin@gmail.com>
> Cc: x86@kernel.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(-)
>=20
> 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;
>=20

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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