Re: [PATCH v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP

[Song Liu <songliubraving@fb.com>]

> On Aug 1, 2019, at 7:50 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> On 07/31, Song Liu wrote:
>> 
>> +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>> +					 unsigned long addr)
>> +{
>> +	struct mm_slot *mm_slot;
>> +	int ret = 0;
>> +
>> +	/* hold mmap_sem for khugepaged_test_exit() */
>> +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
>> +	VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
>> +
>> +	if (unlikely(khugepaged_test_exit(mm)))
>> +		return 0;
>> +
>> +	if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) &&
>> +	    !test_bit(MMF_DISABLE_THP, &mm->flags)) {
>> +		ret = __khugepaged_enter(mm);
>> +		if (ret)
>> +			return ret;
>> +	}
> 
> could you explain why do we need mm->mmap_sem, khugepaged_test_exit() check
> and __khugepaged_enter() ?

If the mm doesn't have a mm_slot, we would like to create one here (by 
calling __khugepaged_enter()). 

This happens when the THP is created by another mm, or by tmpfs with 
"huge=always"; and then page table of this mm got split by split_huge_pmd(). 
With current kernel, this happens when we attach/detach uprobe to a file 
in tmpfs with huge=always. 

Does this answer your question?

Thanks,
Song