Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

Yang Shi
2019-05-09T21:48:45Z

On 5/9/19 2:06 PM, Jan Stancek wrote:
> ----- Original Message -----
>>
>> On 5/9/19 11:24 AM, Peter Zijlstra wrote:
>>> On Thu, May 09, 2019 at 05:36:29PM +0000, Nadav Amit wrote:
>>>>> On May 9, 2019, at 3:38 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>>>>> index 99740e1dd273..fe768f8d612e 100644
>>>>> --- a/mm/mmu_gather.c
>>>>> +++ b/mm/mmu_gather.c
>>>>> @@ -244,15 +244,20 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>>>>> unsigned long start, unsigned long end)
>>>>> {
>>>>> /*
>>>>> - * If there are parallel threads are doing PTE changes on same range
>>>>> - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
>>>>> - * flush by batching, a thread has stable TLB entry can fail to flush
>>>>> - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
>>>>> - * forcefully if we detect parallel PTE batching threads.
>>>>> + * Sensible comment goes here..
>>>>> */
>>>>> - if (mm_tlb_flush_nested(tlb->mm)) {
>>>>> - __tlb_reset_range(tlb);
>>>>> - __tlb_adjust_range(tlb, start, end - start);
>>>>> + if (mm_tlb_flush_nested(tlb->mm) && !tlb->full_mm) {
>>>>> + /*
>>>>> + * Since we're can't tell what we actually should have
>>>>> + * flushed flush everything in the given range.
>>>>> + */
>>>>> + tlb->start = start;
>>>>> + tlb->end = end;
>>>>> + tlb->freed_tables = 1;
>>>>> + tlb->cleared_ptes = 1;
>>>>> + tlb->cleared_pmds = 1;
>>>>> + tlb->cleared_puds = 1;
>>>>> + tlb->cleared_p4ds = 1;
>>>>> }
>>>>>
>>>>> tlb_flush_mmu(tlb);
>>>> As a simple optimization, I think it is possible to hold multiple nesting
>>>> counters in the mm, similar to tlb_flush_pending, for freed_tables,
>>>> cleared_ptes, etc.
>>>>
>>>> The first time you set tlb->freed_tables, you also atomically increase
>>>> mm->tlb_flush_freed_tables. Then, in tlb_flush_mmu(), you just use
>>>> mm->tlb_flush_freed_tables instead of tlb->freed_tables.
>>> That sounds fraught with races and expensive; I would much prefer to not
>>> go there for this arguably rare case.
>>>
>>> Consider such fun cases as where CPU-0 sees and clears a PTE, CPU-1
>>> races and doesn't see that PTE. Therefore CPU-0 sets and counts
>>> cleared_ptes. Then if CPU-1 flushes while CPU-0 is still in mmu_gather,
>>> it will see cleared_ptes count increased and flush that granularity,
>>> OTOH if CPU-1 flushes after CPU-0 completes, it will not and potentiall
>>> miss an invalidate it should have had.
>>>
>>> This whole concurrent mmu_gather stuff is horrible.
>>>
>>> /me ponders more....
>>>
>>> So I think the fundamental race here is this:
>>>
>>> CPU-0 CPU-1
>>>
>>> tlb_gather_mmu(.start=1, tlb_gather_mmu(.start=2,
>>> .end=3); .end=4);
>>>
>>> ptep_get_and_clear_full(2)
>>> tlb_remove_tlb_entry(2);
>>> __tlb_remove_page();
>>> if (pte_present(2)) // nope
>>>
>>> tlb_finish_mmu();
>>>
>>> // continue without TLBI(2)
>>> // whoopsie
>>>
>>> tlb_finish_mmu();
>>> tlb_flush() -> TLBI(2)
>> I'm not quite sure if this is the case Jan really met. But, according to
>> his test, once correct tlb->freed_tables and tlb->cleared_* are set, his
>> test works well.
> My theory was following sequence:
>
> t1: map_write_unmap() t2: dummy()
>
> map_address = mmap()
> map_address[i] = 'b'
> munmap(map_address)
> downgrade_write(&mm->mmap_sem);
> unmap_region()
> tlb_gather_mmu()
> inc_tlb_flush_pending(tlb->mm);
> free_pgtables()
> tlb->freed_tables = 1
> tlb->cleared_pmds = 1
>
> pthread_exit()
> madvise(thread_stack, 8M, MADV_DONTNEED)

I'm not quite familiar with the implementation detail of pthread_exit(),
does pthread_exit() call MADV_DONTNEED all the time? I don't see your
test call it. If so this pattern is definitely possible.

> zap_page_range()
> tlb_gather_mmu()
> inc_tlb_flush_pending(tlb->mm);
>
> tlb_finish_mmu()
> if (mm_tlb_flush_nested(tlb->mm))
> __tlb_reset_range()
> tlb->freed_tables = 0
> tlb->cleared_pmds = 0
> __flush_tlb_range(last_level = 0)
> ...
> map_address = mmap()
> map_address[i] = 'b'
> <page fault loop>
> # PTE appeared valid to me,
> # so I suspected stale TLB entry at higher level as result of "freed_tables = 0"
>
>
> I'm happy to apply/run any debug patches to get more data that would help.
>
>>>
>>> And we can fix that by having tlb_finish_mmu() sync up. Never let a
>>> concurrent tlb_finish_mmu() complete until all concurrenct mmu_gathers
>>> have completed.
>> Not sure if this will scale well.
>>
>>> This should not be too hard to make happen.
>>