> On Aug 8, 2019, at 9:37 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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
