Good Catch.

The original code looks too old, use the ksm_mmlist_lock to protect the mm_list 
looks will affect the performance,
Should we use the RCU to protect the list and not free the mm until out of the 
rcu critical period? 


On 2016/5/6 5:57, Andrea Arcangeli wrote:
> Hello Zhou,
> 
> Great catch.
> 
> On Thu, May 05, 2016 at 08:42:56PM +0800, Zhou Chengming wrote:
>>      remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
>> +    up_read(&mm->mmap_sem);
>>  
>>      spin_lock(&ksm_mmlist_lock);
>>      ksm_scan.mm_slot = list_entry(slot->mm_list.next,
>> @@ -1666,16 +1667,12 @@ next_mm:
>>               */
>>              hash_del(&slot->link);
>>              list_del(&slot->mm_list);
>> -            spin_unlock(&ksm_mmlist_lock);
>>  
>>              free_mm_slot(slot);
>>              clear_bit(MMF_VM_MERGEABLE, &mm->flags);
>> -            up_read(&mm->mmap_sem);
>>              mmdrop(mm);
> 
> I thought the mmap_sem for reading prevented a race of the above
> clear_bit against a concurrent madvise(MADV_MERGEABLE) which takes the
> mmap_sem for writing. After this change can't __ksm_enter run
> concurrently with the clear_bit above introducing a different SMP race
> condition?
> 
>> -    } else {
>> -            spin_unlock(&ksm_mmlist_lock);
>> -            up_read(&mm->mmap_sem);
> 
> The strict obviously safe fix is just to invert the above two,
> up_read; spin_unlock.
> 
> Then I found another instance of this same SMP race condition in
> unmerge_and_remove_all_rmap_items() that you didn't fix.
> 
> Actually for the other instance of the bug the implementation above
> that releases the mmap_sem early sounds safe, because it's a
> ksm_text_exit that takes the clear_bit path, not just the fact we
> didn't find a vma with VM_MERGEABLE set and we garbage collect the
> mm_slot, while the "mm" may still alive. In the other case the "mm"
> isn't alive anymore so the race with MADV_MERGEABLE shouldn't be
> possible to materialize.
> 
> Could you fix it by just inverting the up_read/spin_unlock order, in
> the place you patched, and add this comment:
> 
>       } else {
>               /*
>                * up_read(&mm->mmap_sem) first because after
>                * spin_unlock(&ksm_mmlist_lock) run, the "mm" may
>                * already have been freed under us by __ksm_exit()
>                * because the "mm_slot" is still hashed and
>                * ksm_scan.mm_slot doesn't point to it anymore.
>                */
>               up_read(&mm->mmap_sem);
>               spin_unlock(&ksm_mmlist_lock);
>       }
> 
> And in unmerge_and_remove_all_rmap_items() same thing, except there
> you can apply your up_read() early and you can just drop the "else"
> clause.
> 
> .
> 


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