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

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Message-Id: <0468E994-273E-4A8B-A521-150723DA9774@broadcom.com>
Date: Thu, 4 Jan 2024 15:26:43 +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 Jan 2, 2024, at 4:41 AM, Jisheng Zhang <jszhang@...nel.org> wrote:
> 
> On Sat, Dec 30, 2023 at 11:54:02AM +0200, Nadav Amit wrote:
> 
>> 
>> 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.
> 
> 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’t 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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