Re: [PATCH v12 3/6] mm, thp: introduce FOLL_SPLIT_PMD

[Song Liu <songliubraving@fb.com>]

> On Aug 8, 2019, at 9:37 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> On 08/07, Song Liu wrote:
>> 
>> @@ -399,7 +399,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>> 		spin_unlock(ptl);
>> 		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
>> 	}
>> -	if (flags & FOLL_SPLIT) {
>> +	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
>> 		int ret;
>> 		page = pmd_page(*pmd);
>> 		if (is_huge_zero_page(page)) {
>> @@ -408,7 +408,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>> 			split_huge_pmd(vma, pmd, address);
>> 			if (pmd_trans_unstable(pmd))
>> 				ret = -EBUSY;
>> -		} else {
>> +		} else if (flags & FOLL_SPLIT) {
>> 			if (unlikely(!try_get_page(page))) {
>> 				spin_unlock(ptl);
>> 				return ERR_PTR(-ENOMEM);
>> @@ -420,6 +420,10 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>> 			put_page(page);
>> 			if (pmd_none(*pmd))
>> 				return no_page_table(vma, flags);
>> +		} else {  /* flags & FOLL_SPLIT_PMD */
>> +			spin_unlock(ptl);
>> +			split_huge_pmd(vma, pmd, address);
>> +			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
>> 		}
> 
> Can't resist, let me repeat that I do not like this patch because imo
> it complicates this code for no reason.

Personally, I don't think this is more complicated than your version. 
This patch is safe as it doesn't change any code for is_huge_zero_page() 
case. 

Also, if some code calls follow_pmd_mask() with flags contains both 
FOLL_SPLIT and FOLL_SPLIT_PMD, we should honor FOLL_SPLIT and split the
huge page. Of course, there is no code that sets both flags.

Does this resolve your concern here?

Thanks,
Song