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

[Will Deacon]

On Thu, Dec 28, 2023 at 04:46:41PM +0800, Jisheng Zhang wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> ---
>  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;

Why isn't the 'last_level' check sufficient here? In other words, when do
we perform a !last_level invalidation with 'fullmm' set outside of teardown?

Will