Re: [PATCH] arch, mm: introduce arch_tlb_gather_mmu_lazy (was: Re: [RESEND PATCH] mm, oom_reaper: gather each vma to prevent) leaking TLB entry

[Will Deacon <will.deacon@arm.com>]

Hi Michal,

On Fri, Nov 10, 2017 at 01:26:35PM +0100, Michal Hocko wrote:
> From 7f0fcd2cab379ddac5611b2a520cdca8a77a235b Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Fri, 10 Nov 2017 11:27:17 +0100
> Subject: [PATCH] arch, mm: introduce arch_tlb_gather_mmu_lazy
> 
> 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1") has
> introduced an optimization to not flush tlb when we are tearing the
> whole address space down. Will goes on to explain
> 
> : Basically, we tag each address space with an ASID (PCID on x86) which
> : is resident in the TLB. This means we can elide TLB invalidation when
> : pulling down a full mm because we won't ever assign that ASID to
> : another mm without doing TLB invalidation elsewhere (which actually
> : just nukes the whole TLB).
> 
> This all is nice but tlb_gather users are not aware of that and this can
> actually cause some real problems. E.g. the oom_reaper tries to reap the
> whole address space but it might race with threads accessing the memory [1].
> It is possible that soft-dirty handling might suffer from the same
> problem [2].
> 
> Introduce an explicit lazy variant tlb_gather_mmu_lazy which allows the
> behavior arm64 implements for the fullmm case and replace it by an
> explicit lazy flag in the mmu_gather structure. exit_mmap path is then
> turned into the explicit lazy variant. Other architectures simply ignore
> the flag.
> 
> [1] http://lkml.kernel.org/r/20171106033651.172368-1-wangnan0@huawei.com
> [2] http://lkml.kernel.org/r/20171110001933.GA12421@bbox
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  arch/arm/include/asm/tlb.h   |  3 ++-
>  arch/arm64/include/asm/tlb.h |  2 +-
>  arch/ia64/include/asm/tlb.h  |  3 ++-
>  arch/s390/include/asm/tlb.h  |  3 ++-
>  arch/sh/include/asm/tlb.h    |  2 +-
>  arch/um/include/asm/tlb.h    |  2 +-
>  include/asm-generic/tlb.h    |  6 ++++--
>  include/linux/mm_types.h     |  2 ++
>  mm/memory.c                  | 17 +++++++++++++++--
>  mm/mmap.c                    |  2 +-
>  10 files changed, 31 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm/include/asm/tlb.h b/arch/arm/include/asm/tlb.h
> index d5562f9ce600..fe9042aee8e9 100644
> --- a/arch/arm/include/asm/tlb.h
> +++ b/arch/arm/include/asm/tlb.h
> @@ -149,7 +149,8 @@ static inline void tlb_flush_mmu(struct mmu_gather *tlb)
>  
>  static inline void
>  arch_tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
> -			unsigned long start, unsigned long end)
> +			unsigned long start, unsigned long end,
> +			bool lazy)
>  {
>  	tlb->mm = mm;
>  	tlb->fullmm = !(start | (end+1));
> diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
> index ffdaea7954bb..7adde19b2bcc 100644
> --- a/arch/arm64/include/asm/tlb.h
> +++ b/arch/arm64/include/asm/tlb.h
> @@ -43,7 +43,7 @@ static inline void tlb_flush(struct mmu_gather *tlb)
>  	 * The ASID allocator will either invalidate the ASID or mark
>  	 * it as used.
>  	 */
> -	if (tlb->fullmm)
> +	if (tlb->lazy)
>  		return;

This looks like the right idea, but I'd rather make this check:

	if (tlb->fullmm && tlb->lazy)

since the optimisation doesn't work for anything than tearing down the
entire address space.

Alternatively, I could actually go check MMF_UNSTABLE in tlb->mm, which
would save you having to add an extra flag in the first place, e.g.:

	if (tlb->fullmm && !test_bit(MMF_UNSTABLE, &tlb->mm->flags))

which is a nice one-liner.

Will