Re: [RFC 08/20] mm: store completed TLB generation

[Nadav Amit <namit@vmware.com>]

> On Jan 31, 2021, at 12:32 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Sat, Jan 30, 2021 at 4:16 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> To detect deferred TLB flushes in fine granularity, we need to keep
>> track on the completed TLB flush generation for each mm.
>> 
>> Add logic to track for each mm the tlb_gen_completed, which tracks the
>> completed TLB generation. It is the arch responsibility to call
>> mark_mm_tlb_gen_done() whenever a TLB flush is completed.
>> 
>> Start the generation numbers from 1 instead of 0. This would allow later
>> to detect whether flushes of a certain generation were completed.
> 
> Can you elaborate on how this helps?

I guess it should have gone to patch 15.

The relevant code it interacts with is in read_defer_tlb_flush_gen(). It
allows to use a single check to see “outdated” deferred TLB gen. Initially
tlb->defer_gen is zero. We are going to do inc_mm_tlb_gen() both on the
first time we defer TLB entries and whenever we see mm_gen is newer than
tlb->defer_gen:

+       mm_gen = atomic64_read(&mm->tlb_gen);
+
+       /*
+        * This condition checks for both first deferred TLB flush and for other
+        * TLB pending or executed TLB flushes after the last table that we
+        * updated. In the latter case, we are going to skip a generation, which
+        * would lead to a full TLB flush. This should therefore not cause
+        * correctness issues, and should not induce overheads, since anyhow in
+        * TLB storms it is better to perform full TLB flush.
+        */
+       if (mm_gen != tlb->defer_gen) {
+               VM_BUG_ON(mm_gen < tlb->defer_gen);
+
+               tlb->defer_gen = inc_mm_tlb_gen(mm);
+       }


> 
> I think you should document that tlb_gen_completed only means that no
> outdated TLB entries will be observably used.  In the x86
> implementation it's possible for older TLB entries to still exist,
> unused, in TLBs of cpus running other mms.

You mean entries that be later flushed during switch_mm_irqs_off(), right? I
think that overall my comments need some work. Yes.

> How does this work with arch_tlbbatch_flush()?

completed_gen is not updated by arch_tlbbatch_flush(), since I couldn’t find
a way to combine them. completed_gen might not catch up with tlb_gen in this
case until another TLB flush takes place. I do not see correctness issue,
but it might result in redundant TLB flush.

>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> 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/x86/mm/tlb.c         | 10 ++++++++++
>> include/asm-generic/tlb.h | 33 +++++++++++++++++++++++++++++++++
>> include/linux/mm_types.h  | 15 ++++++++++++++-
>> 3 files changed, 57 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 7ab21430be41..d17b5575531e 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -14,6 +14,7 @@
>> #include <asm/nospec-branch.h>
>> #include <asm/cache.h>
>> #include <asm/apic.h>
>> +#include <asm/tlb.h>
>> 
>> #include "mm_internal.h"
>> 
>> @@ -915,6 +916,9 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>>        if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
>>                flush_tlb_others(mm_cpumask(mm), info);
>> 
>> +       /* Update the completed generation */
>> +       mark_mm_tlb_gen_done(mm, new_tlb_gen);
>> +
>>        put_flush_tlb_info();
>>        put_cpu();
>> }
>> @@ -1147,6 +1151,12 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
>> 
>>        cpumask_clear(&batch->cpumask);
>> 
>> +       /*
>> +        * We cannot call mark_mm_tlb_gen_done() since we do not know which
>> +        * mm's should be flushed. This may lead to some unwarranted TLB
>> +        * flushes, but not to correction problems.
>> +        */
>> +
>>        put_cpu();
>> }
>> 
>> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
>> index 517c89398c83..427bfcc6cdec 100644
>> --- a/include/asm-generic/tlb.h
>> +++ b/include/asm-generic/tlb.h
>> @@ -513,6 +513,39 @@ static inline void tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vm
>> }
>> #endif
>> 
>> +#ifdef CONFIG_ARCH_HAS_TLB_GENERATIONS
>> +
>> +/*
>> + * Helper function to update a generation to have a new value, as long as new
>> + * value is greater or equal to gen.
>> + */
> 
> I read this a couple of times, and I don't understand it.  How about:
> 
> Helper function to atomically set *gen = max(*gen, new_gen)
> 
>> +static inline void tlb_update_generation(atomic64_t *gen, u64 new_gen)
>> +{
>> +       u64 cur_gen = atomic64_read(gen);
>> +
>> +       while (cur_gen < new_gen) {
>> +               u64 old_gen = atomic64_cmpxchg(gen, cur_gen, new_gen);
>> +
>> +               /* Check if we succeeded in the cmpxchg */
>> +               if (likely(cur_gen == old_gen))
>> +                       break;
>> +
>> +               cur_gen = old_gen;
>> +       };
>> +}
>> +
>> +
>> +static inline void mark_mm_tlb_gen_done(struct mm_struct *mm, u64 gen)
>> +{
>> +       /*
>> +        * Update the completed generation to the new generation if the new
>> +        * generation is greater than the previous one.
>> +        */
>> +       tlb_update_generation(&mm->tlb_gen_completed, gen);
>> +}
>> +
>> +#endif /* CONFIG_ARCH_HAS_TLB_GENERATIONS */
>> +
>> /*
>>  * tlb_flush_{pte|pmd|pud|p4d}_range() adjust the tlb->start and tlb->end,
>>  * and set corresponding cleared_*.
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 2035ac319c2b..8a5eb4bfac59 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -571,6 +571,13 @@ struct mm_struct {
>>                 * This is not used on Xen PV.
>>                 */
>>                atomic64_t tlb_gen;
>> +
>> +               /*
>> +                * TLB generation which is guarnateed to be flushed, including
> 
> guaranteed
> 
>> +                * all the PTE changes that were performed before tlb_gen was
>> +                * incremented.
>> +                */
> 
> I will defer judgment to future patches before I believe that this isn't racy :)

Fair enough. Thanks for the review.