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

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Date: Wed, 3 Jan 2024 17:57:43 +0000
From: Will Deacon <will@...nel.org>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: "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@...ck.org,
	linux-arm-kernel@...ts.infradead.org, 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>,
	x86@...nel.org
Subject: Re: [PATCH 1/2] mm/tlb: fix fullmm semantics

On Wed, Jan 03, 2024 at 05:50:01PM +0000, Will Deacon wrote:
> On Thu, Dec 28, 2023 at 04:46:41PM +0800, Jisheng Zhang wrote:
> > From: Nadav Amit <namit@...are.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@...are.com>
> > Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Andy Lutomirski <luto@...nel.org>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Yu Zhao <yuzhao@...gle.com>
> > Cc: Nick Piggin <npiggin@...il.com>
> > Cc: x86@...nel.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?

Sorry, logic inversion typo there. I should've said:

  When do we perform a last_level invalidation with 'fullmm' set outside
  of teardown?

I remember this used to be the case for OOM ages ago, but 687cb0884a71
("mm, oom_reaper: gather each vma to prevent leaking TLB entry") sorted
that out.

I'm not against making this clearer and/or more robust, I'm just trying
to understand whether this is fixing a bug (as implied by the subject)
or not.

Will