linux-kernel - Re: [PATCH 1/8] lazy tlb: introduce exit_lazy

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Message-Id: <1606876111.f9oqzur49r.astroid@bobo.none>
Date:   Wed, 02 Dec 2020 12:49:58 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Anton Blanchard <anton@...abs.org>, Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 1/8] lazy tlb: introduce exit_lazy_tlb

Excerpts from Andy Lutomirski's message of November 29, 2020 10:38 am:
> On Sat, Nov 28, 2020 at 8:01 AM Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> This is called at points where a lazy mm is switched away or made not
>> lazy (by its owner switching back).
>>
>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>> ---
>>  arch/arm/mach-rpc/ecard.c            |  1 +
>>  arch/powerpc/mm/book3s64/radix_tlb.c |  1 +
>>  fs/exec.c                            |  6 ++++--
>>  include/asm-generic/mmu_context.h    | 21 +++++++++++++++++++++
>>  kernel/kthread.c                     |  1 +
>>  kernel/sched/core.c                  |  2 ++
>>  6 files changed, 30 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/mach-rpc/ecard.c b/arch/arm/mach-rpc/ecard.c
>> index 827b50f1c73e..43eb1bfba466 100644
>> --- a/arch/arm/mach-rpc/ecard.c
>> +++ b/arch/arm/mach-rpc/ecard.c
>> @@ -253,6 +253,7 @@ static int ecard_init_mm(void)
>>         current->mm = mm;
>>         current->active_mm = mm;
>>         activate_mm(active_mm, mm);
>> +       exit_lazy_tlb(active_mm, current);
>>         mmdrop(active_mm);
>>         ecard_init_pgtables(mm);
>>         return 0;
>> diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
>> index b487b489d4b6..ac3fec03926a 100644
>> --- a/arch/powerpc/mm/book3s64/radix_tlb.c
>> +++ b/arch/powerpc/mm/book3s64/radix_tlb.c
>> @@ -661,6 +661,7 @@ static void do_exit_flush_lazy_tlb(void *arg)
>>                 mmgrab(&init_mm);
>>                 current->active_mm = &init_mm;
>>                 switch_mm_irqs_off(mm, &init_mm, current);
>> +               exit_lazy_tlb(mm, current);
>>                 mmdrop(mm);
>>         }
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 547a2390baf5..4b4dea1bb7ba 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1017,6 +1017,8 @@ static int exec_mmap(struct mm_struct *mm)
>>         if (!IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM))
>>                 local_irq_enable();
>>         activate_mm(active_mm, mm);
>> +       if (!old_mm)
>> +               exit_lazy_tlb(active_mm, tsk);
>>         if (IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM))
>>                 local_irq_enable();
>>         tsk->mm->vmacache_seqnum = 0;
>> @@ -1028,9 +1030,9 @@ static int exec_mmap(struct mm_struct *mm)
>>                 setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
>>                 mm_update_next_owner(old_mm);
>>                 mmput(old_mm);
>> -               return 0;
>> +       } else {
>> +               mmdrop(active_mm);
>>         }
>> -       mmdrop(active_mm);
> 
> This looks like an unrelated change.

I thought the old style was pointless and made me look twice to make 
sure we weren't mmdrop()ing the lazy.

> 
>>         return 0;
>>  }
>>
>> diff --git a/include/asm-generic/mmu_context.h b/include/asm-generic/mmu_context.h
>> index 91727065bacb..4626d0020e65 100644
>> --- a/include/asm-generic/mmu_context.h
>> +++ b/include/asm-generic/mmu_context.h
>> @@ -24,6 +24,27 @@ static inline void enter_lazy_tlb(struct mm_struct *mm,
>>  }
>>  #endif
>>
>> +/*
>> + * exit_lazy_tlb - Called after switching away from a lazy TLB mode mm.
>> + *
>> + * mm:  the lazy mm context that was switched
>> + * tsk: the task that was switched to (with a non-lazy mm)
>> + *
>> + * mm may equal tsk->mm.
>> + * mm and tsk->mm will not be NULL.
>> + *
>> + * Note this is not symmetrical to enter_lazy_tlb, this is not
>> + * called when tasks switch into the lazy mm, it's called after the
>> + * lazy mm becomes non-lazy (either switched to a different mm or the
>> + * owner of the mm returns).
>> + */
>> +#ifndef exit_lazy_tlb
>> +static inline void exit_lazy_tlb(struct mm_struct *mm,
> 
> Maybe name this parameter prev_lazy_mm?
> 

mm is better because it's the mm that we're "exiting lazy" from, the 
function name gives the context.

prev might suggest it was the previous but it's the current one, or
that we're switching to another mm but we may not be at all.

Thanks,
Nick