> On Jul 30, 2019, at 9:11 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> I don't understand this code, so I can't review, but.
>
> On 07/29, Song Liu wrote:
>>
>> This patches introduces a new foll_flag: FOLL_SPLIT_PMD. As the name says
>> FOLL_SPLIT_PMD splits huge pmd for given mm_struct, the underlining huge
>> page stays as-is.
>>
>> FOLL_SPLIT_PMD is useful for cases where we need to use regular pages,
>> but would switch back to huge page and huge pmd on. One of such example
>> is uprobe. The following patches use FOLL_SPLIT_PMD in uprobe.
>
> So after the next patch we have a single user of FOLL_SPLIT_PMD (uprobes)
> and a single user of FOLL_SPLIT: arch/s390/mm/gmap.c:thp_split_mm().
>
> Hmm.
I think this is what we want. :)
FOLL_SPLIT is the fallback solution for users who cannot handle THP. With
more THP aware code, there will be fewer users of FOLL_SPLIT.
>
>> @@ -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);
>
> I fail to understand why this differs from the is_huge_zero_page() case above.
split_huge_pmd() handles is_huge_zero_page() differently. In this case, we
cannot use the pmd_trans_unstable() check.
>
> Anyway, ret = pte_alloc(mm, pmd) can't be correct. If __pte_alloc() fails pte_alloc()
> will return 1. This will fool the IS_ERR(page) check in __get_user_pages().
Great catch! Let me fix it.
Thanks,
Song
