Re: [PATCH v3 1/2] khugepaged: enable collapse pmd for pte-mapped THP

[Song Liu <songliubraving@fb.com>]

> On Aug 2, 2019, at 9:21 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> On 08/01, 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;
>> +	}
> 
> see my reply to v2
> 
>> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long haddr)
>> +{
>> +	struct vm_area_struct *vma = find_vma(mm, haddr);
>> +	pmd_t *pmd = mm_find_pmd(mm, haddr);
>> +	struct page *hpage = NULL;
>> +	unsigned long addr;
>> +	spinlock_t *ptl;
>> +	int count = 0;
>> +	pmd_t _pmd;
>> +	int i;
>> +
>> +	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
>> +
>> +	if (!vma || !vma->vm_file || !pmd)
>                    ^^^^^^^^^^^^^
> 
> I am not sure this is enough,
> 
>> +		return;
>> +
>> +	/* step 1: check all mapped PTEs are to the right huge page */
>> +	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
>> +		pte_t *pte = pte_offset_map(pmd, addr);
>> +		struct page *page;
>> +
>> +		if (pte_none(*pte))
>> +			continue;
>> +
>> +		page = vm_normal_page(vma, addr, *pte);
> 
> Why can't vm_normal_page() return NULL? Again, we do not if this vm_file
> is the same shmem_file() or something else.

Hmm.. I guess we also need to check page != NULL. 

For vm_file, I guess it is OK for it to be another file. As long as all
pages in the pmd map to the same THP, we should be fine. 

> 
> And in fact I don't think it is safe to use vm_normal_page(vma, addr)
> unless you know that vma includes this addr.

Yeah, we need to check vma includes this address. 

> 
> to be honest, I am not even sure that unconditional mm_find_pmd() is safe
> if this "something else" is really special.

I cannot imagine why mm_find_pmd() could be unsafe. 

Thanks,
Song