Re: [PATCH v3 07/12] mm/gup: track FOLL_PIN pages

John Hubbard
2020-02-03T21:01:18Z

On 2/3/20 5:40 AM, Kirill A. Shutemov wrote:
> On Fri, Jan 31, 2020 at 07:40:24PM -0800, John Hubbard wrote:
>> @@ -4405,7 +4392,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>> same_page:
>> if (pages) {
>> pages[i] = mem_map_offset(page, pfn_offset);
>> - get_page(pages[i]);
>> + if (!try_grab_page(pages[i], flags)) {
>> + spin_unlock(ptl);
>> + remainder = 0;
>> + err = -ENOMEM;
>> + WARN_ON_ONCE(1);
>
> The WARN_ON_ONCE deserve a comment. And I guess you can put it into 'if'
> condition.

OK, I've changed it to the following, which I *think* is an accurate comment, but
I'm still a bit new to huge pages:

if (pages) {
pages[i] = mem_map_offset(page, pfn_offset);
/*
* try_grab_page() should always succeed here, because:
* a) we hold the ptl lock, and b) we've just checked
* that the huge page is present in the page tables. If
* the huge page is present, then the tail pages must
* also be present. The ptl prevents the head page and
* tail pages from being rearranged in any way. So this
* page must be available at this point, unless the page
* refcount overflowed:
*/
if (WARN_ON_ONCE(!try_grab_page(pages[i], flags))) {
spin_unlock(ptl);
remainder = 0;
err = -ENOMEM;
break;
}
}

>
>> + break;
>> + }
>> }
>>
>> if (vmas)
>> @@ -4965,6 +4958,12 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>> struct page *page = NULL;
>> spinlock_t *ptl;
>> pte_t pte;
>> +
>> + /* FOLL_GET and FOLL_PIN are mutually exclusive. */
>> + if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
>> + (FOLL_PIN | FOLL_GET)))
>> + return NULL;
>> +
>> retry:
>> ptl = pmd_lockptr(mm, pmd);
>> spin_lock(ptl);
>> @@ -4977,8 +4976,11 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>> pte = huge_ptep_get((pte_t *)pmd);
>> if (pte_present(pte)) {
>> page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
>> - if (flags & FOLL_GET)
>> - get_page(page);
>> + if (unlikely(!try_grab_page(page, flags))) {
>> + WARN_ON_ONCE(1);
>
> Ditto.

OK, I've added a similar comment as the one above. Now it looks like this:

if (pte_present(pte)) {
page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
/*
* try_grab_page() should always succeed here, because: a) we
* hold the pmd (ptl) lock, and b) we've just checked that the
* huge pmd (head) page is present in the page tables. The ptl
* prevents the head page and tail pages from being rearranged
* in any way. So this page must be available at this point,
* unless the page refcount overflowed:
*/
if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
page = NULL;
goto out;
}

thanks,
--
John Hubbard
NVIDIA

>
>> + page = NULL;
>> + goto out;
>> + }
>> } else {
>> if (is_hugetlb_entry_migration(pte)) {
>> spin_unlock(ptl);
>