Re: [PATCH] mm: Prevent racy access to tlb_flush_pending

[Nadav Amit <nadav.amit@gmail.com>]

Nadav Amit <nadav.amit@gmail.com> wrote:

> Andrew Morton <akpm@linux-foundation.org> wrote:
> 
>> On Mon, 17 Jul 2017 11:02:46 -0700 Nadav Amit <namit@vmware.com> wrote:
>> 
>>> Setting and clearing mm->tlb_flush_pending can be performed by multiple
>>> threads, since mmap_sem may only be acquired for read in task_numa_work.
>>> If this happens, tlb_flush_pending may be cleared while one of the
>>> threads still changes PTEs and batches TLB flushes.
>>> 
>>> As a result, TLB flushes can be skipped because the indication of
>>> pending TLB flushes is lost, for instance due to race between
>>> migration and change_protection_range (just as in the scenario that
>>> caused the introduction of tlb_flush_pending).
>>> 
>>> The feasibility of such a scenario was confirmed by adding assertion to
>>> check tlb_flush_pending is not set by two threads, adding artificial
>>> latency in change_protection_range() and using sysctl to reduce
>>> kernel.numa_balancing_scan_delay_ms.
>>> 
>>> Fixes: 20841405940e ("mm: fix TLB flush race between migration, and
>>> change_protection_range")
>> 
>> The changelog doesn't describe the user-visible effects of the bug (it
>> should always do so, please).  But it is presumably a data-corruption
>> bug so I suggest that a -stable backport is warranted?
> 
> Yes, although I did not encounter an actual memory corruption.
> 
>> It has been there for 4 years so I'm thinking we can hold off a
>> mainline (and hence -stable) merge until 4.13-rc1, yes?
>> 
>> 
>> One thought:
>> 
>>> --- a/include/linux/mm_types.h
>>> +++ b/include/linux/mm_types.h
>>> 
>>> ...
>>> 
>>> @@ -528,11 +528,11 @@ static inline cpumask_t *mm_cpumask(struct mm_struct *mm)
>>> static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
>>> {
>>> 	barrier();
>>> -	return mm->tlb_flush_pending;
>>> +	return atomic_read(&mm->tlb_flush_pending) > 0;
>>> }
>>> static inline void set_tlb_flush_pending(struct mm_struct *mm)
>>> {
>>> -	mm->tlb_flush_pending = true;
>>> +	atomic_inc(&mm->tlb_flush_pending);
>>> 
>>> 	/*
>>> 	 * Guarantee that the tlb_flush_pending store does not leak into the
>>> @@ -544,7 +544,7 @@ static inline void set_tlb_flush_pending(struct mm_struct *mm)
>>> static inline void clear_tlb_flush_pending(struct mm_struct *mm)
>>> {
>>> 	barrier();
>>> -	mm->tlb_flush_pending = false;
>>> +	atomic_dec(&mm->tlb_flush_pending);
>>> }
>>> #else
>> 
>> Do we still need the barrier()s or is it OK to let the atomic op do
>> that for us (with a suitable code comment).
> 
> I will submit v2. However, I really don’t understand the comment on
> mm_tlb_flush_pending():
> 
> /*              
> * Memory barriers to keep this state in sync are graciously provided by
> * the page table locks, outside of which no page table modifications happen.
> * The barriers below prevent the compiler from re-ordering the instructions
> * around the memory barriers that are already present in the code.
> */
> 
> But IIUC migrate_misplaced_transhuge_page() does not call
> mm_tlb_flush_pending() while the ptl is taken.
> 
> Mel, can I bother you again? Should I move the flush in
> migrate_misplaced_transhuge_page() till after the ptl is taken?

Oops: this would be obviously wrong since it would move it after
migrate_page_copy() is run. So I do need your advice whether the comment is
wrong or the implementation.

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