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

[Nadav Amit <nadav.amit@broadcom.com>]

> On Jan 2, 2024, at 4:41 AM, Jisheng Zhang <jszhang@kernel.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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