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

[Song Liu <songliubraving@fb.com>]

> On Aug 9, 2019, at 9:35 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> On 08/08, Song Liu wrote:
>> 
>>> 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.
> 
> I do, but of course this is subjective.
> 
>> 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.
> 
> Heh. why not other way around?

Because FOLL_SPLIT splits both the page and the pmd. FOLL_SPLIT_PMD 
only splits the pmd, so it is a subset of FOLL_SPLIT. When the user
sets both, we should split both the page and the pmd. 

Thanks,
Song