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

[John Hubbard]

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);
>