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Subject: Re: [PATCH 1/2] mm/tlb: fix fullmm semantics
From: Nadav Amit <nadav.amit@broadcom.com>
In-Reply-To: <ZZN35DTJTNExCNXW@xhacker>
Date: Thu, 4 Jan 2024 15:26:43 +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: <0468E994-273E-4A8B-A521-150723DA9774@broadcom.com>
References: <20231228084642.1765-1-jszhang@kernel.org>
 <20231228084642.1765-2-jszhang@kernel.org>
 <204B6410-2EFA-462B-9DF7-64CC5F1D3AD2@broadcom.com>
 <ZZN35DTJTNExCNXW@xhacker>
To: Jisheng Zhang <jszhang@kernel.org>
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> On Jan 2, 2024, at 4:41=E2=80=AFAM, Jisheng Zhang <jszhang@kernel.org> wr=
ote:
>=20
> On Sat, Dec 30, 2023 at 11:54:02AM +0200, Nadav Amit wrote:
>=20
>>=20
>> 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.
>=20
> will do if the above change is accepted by arm64

Jisheng, I expected somebody with arm64 knowledge to point it out, and
maybe I am wrong, but I really don=E2=80=99t understand something about the
correctness, if you can please explain.

In the following code:

--- 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;

You skip flush if fullmm is on. But if page-tables are freed, you may
want to flush immediately and not wait for ASID to be freed to avoid
speculative page walks; these walks at least on x86 caused a mess.

No?


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