Re: [PATCH v3] mm/hugetlb: fix memory offline with hugepage size > memory block size

Mike Kravetz
2016-09-22T19:14:45Z

On 09/22/2016 11:12 AM, Dave Hansen wrote:
> On 09/22/2016 09:29 AM, Gerald Schaefer wrote:
>> static void dissolve_free_huge_page(struct page *page)
>> {
>> + struct page *head = compound_head(page);
>> + struct hstate *h = page_hstate(head);
>> + int nid = page_to_nid(head);
>> +
>> spin_lock(&hugetlb_lock);
>> - if (PageHuge(page) && !page_count(page)) {
>> - struct hstate *h = page_hstate(page);
>> - int nid = page_to_nid(page);
>> - list_del(&page->lru);
>> - h->free_huge_pages--;
>> - h->free_huge_pages_node[nid]--;
>> - h->max_huge_pages--;
>> - update_and_free_page(h, page);
>> - }
>> + list_del(&head->lru);
>> + h->free_huge_pages--;
>> + h->free_huge_pages_node[nid]--;
>> + h->max_huge_pages--;
>> + update_and_free_page(h, head);
>> spin_unlock(&hugetlb_lock);
>> }
>
> Do you need to revalidate anything once you acquire the lock? Can this,
> for instance, race with another thread doing vm.nr_hugepages=0? Or a
> thread faulting in and allocating the large page that's being dissolved?

I originally suggested the locking change, but this is not quite right.
The page count for huge pages is adjusted while holding hugetlb_lock.
So, that check or a revalidation needs to be done while holding the lock.

That question made me think about huge page reservations. I don't think
the offline code takes this into account. But, you would not want your
huge page count to drop below the reserved huge page count
(resv_huge_pages).
So, shouldn't this be another condition to check before allowing the huge
page to be dissolved?

--
Mike Kravetz