RE: [RFC PATCH 3/3] Change THP behavior

Kirill A. Shutemov
2013-12-13T13:13:59Z

Alex Thorlton wrote:
> + /*
> + * now that we've done the accounting work, we check to see if
> + * we've exceeded our threshold
> + */
> + if (temp_thp->ref_count >= mm->thp_threshold) {
> + pmd_t pmd_entry;
> + pgtable_t pgtable;
> +
> + /*
> + * we'll do all of the following beneath the big ptl for now
> + * this will need to be modified to work with the split ptl
> + */
> + spin_lock(&mm->page_table_lock);
> +
> + /*
> + * once we get past the lock we have to make sure that somebody
> + * else hasn't already turned this guy into a THP, if they have,
> + * then the page we need is already faulted in as part of the THP
> + * they created
> + */
> + if (PageTransHuge(temp_thp->page)) {
> + spin_unlock(&mm->page_table_lock);
> + return 0;
> + }
> +
> + pgtable = pte_alloc_one(mm, haddr);
> + if (unlikely(!pgtable)) {
> + spin_unlock(&mm->page_table_lock);
> + return VM_FAULT_OOM;
> + }
> +
> + /* might wanna move this? */
> + __SetPageUptodate(temp_thp->page);
> +
> + /* turn the pages into one compound page */
> + make_compound_page(temp_thp->page, HPAGE_PMD_ORDER);
> +
> + /* set up the pmd */
> + pmd_entry = mk_huge_pmd(temp_thp->page, vma->vm_page_prot);
> + pmd_entry = maybe_pmd_mkwrite(pmd_mkdirty(pmd_entry), vma);
> +
> + /* remap the new page since we cleared the mappings */
> + page_add_anon_rmap(temp_thp->page, vma, address);
> +
> + /* deposit the thp */
> + pgtable_trans_huge_deposit(mm, pmd, pgtable);
> +
> + set_pmd_at(mm, haddr, pmd, pmd_entry);
> + add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR - mm->thp_threshold + 1);
> + /* mm->nr_ptes++; */
> +
> + /* delete the reference to this compound page from our list */
> + spin_lock(&mm->thp_list_lock);
> + list_del(&temp_thp->list);
> + spin_unlock(&mm->thp_list_lock);
> +
> + spin_unlock(&mm->page_table_lock);
> + return 0;

Hm. I think this part is not correct: you collapse temp thp page
into real one only for current procees. What will happen if a process with
temp thp pages was forked?

And I don't think this problem is an easy one. khugepaged can't collapse
pages with page->_count != 1 for the same reason: to make it properly you
need to take mmap_sem for all processes and collapse all pages at once.
And if a page is pinned, we also can't collapse.

Sorry, I don't think the whole idea has much potential. :(

--
Kirill A. Shutemov