linux-kernel - Re: [PATCH] x86/mm: Change barriers before TLB flushes to smp_mb__after

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Date:	Fri, 15 Jul 2016 11:43:45 -0700
From:	Nadav Amit <nadav.amit@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nadav Amit <namit@...are.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Michal Hocko <mhocko@...e.com>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH] x86/mm: Change barriers before TLB flushes to smp_mb__after_atomic

Andy Lutomirski <luto@...capital.net> wrote:

> On Fri, May 27, 2016 at 8:16 PM, Nadav Amit <namit@...are.com> wrote:
>> When (current->active_mm != mm), flush_tlb_page() does not perform a
>> memory barrier. In practice, this memory barrier is not needed since in
>> the existing call-sites the PTE is modified using atomic-operations.
>> This patch therefore modifies the existing smp_mb in flush_tlb_page to
>> smp_mb__after_atomic and adds the missing one, while documenting the new
>> assumption of flush_tlb_page.
>> 
>> In addition smp_mb__after_atomic is also added to
>> set_tlb_ubc_flush_pending, since it makes a similar implicit assumption
>> and omits the memory barrier.
>> 
>> Signed-off-by: Nadav Amit <namit@...are.com>
>> ---
>> arch/x86/mm/tlb.c | 9 ++++++++-
>> mm/rmap.c         | 3 +++
>> 2 files changed, 11 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index fe9b9f7..2534333 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -242,6 +242,10 @@ out:
>>        preempt_enable();
>> }
>> 
>> +/*
>> + * Calls to flush_tlb_page must be preceded by atomic PTE change or
>> + * explicit memory-barrier.
>> + */
>> void flush_tlb_page(struct vm_area_struct *vma, unsigned long start)
>> {
>>        struct mm_struct *mm = vma->vm_mm;
>> @@ -259,8 +263,11 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long start)
>>                        leave_mm(smp_processor_id());
>> 
>>                        /* Synchronize with switch_mm. */
>> -                       smp_mb();
>> +                       smp_mb__after_atomic();
>>                }
>> +       } else {
>> +               /* Synchronize with switch_mm. */
>> +               smp_mb__after_atomic();
>>        }
>> 
>>        if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids)
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 307b555..60ab0fe 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -613,6 +613,9 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
>> {
>>        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
>> 
>> +       /* Synchronize with switch_mm. */
>> +       smp_mb__after_atomic();
>> +
>>        cpumask_or(&tlb_ubc->cpumask, &tlb_ubc->cpumask, mm_cpumask(mm));
>>        tlb_ubc->flush_required = true;
>> 
>> --
>> 2.7.4
> 
> This looks fine for x86, but I have no idea whether other
> architectures are okay with it.  akpm?  mm folks?

Ping?

Note that this patch adds two missing barriers.

Thanks,
Nadav