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

Will Deacon
2024-01-03T17:50:28Z

On Thu, Dec 28, 2023 at 04:46:41PM +0800, Jisheng Zhang wrote:
> From: Nadav Amit <namit@vmware.com>
>
> 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@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(-)
>
> 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