Re: [PATCH 4/4] hugetlb: Do early cow when page pinned on src mm

On Wed, Feb 03, 2021 at 02:04:30PM -0800, Mike Kravetz wrote:

quoted

@@ -3816,6 +3832,54 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 			}
 			set_huge_swap_pte_at(dst, addr, dst_pte, entry, sz);
 		} else {
+			entry = huge_ptep_get(src_pte);
+			ptepage = pte_page(entry);
+			get_page(ptepage);
+
+			if (unlikely(page_needs_cow_for_dma(vma, ptepage))) {
+				/* This is very possibly a pinned huge page */
+				if (!prealloc) {
+					/*
+					 * Preallocate the huge page without
+					 * tons of locks since we could sleep.
+					 * Note: we can't use any reservation
+					 * because the page will be exclusively
+					 * owned by the child later.
+					 */
+					put_page(ptepage);
+					spin_unlock(src_ptl);
+					spin_unlock(dst_ptl);
+					prealloc = alloc_huge_page(vma, addr, 0);

One quick question:

The comment says we can't use any reservation, and I agree.  However, the
alloc_huge_page call has 0 as the avoid_reserve argument.  Shouldn't that
be !0 to avoid reserves?

Good point..  so I obviously wanted to skip reservation check but successfully
got cheated by the inverted name. :)

Though I do checked the reservation, so it seems not extremely important - when
we fork and copy the vma, we have already dropped the vma resv map:

		if (is_vm_hugetlb_page(tmp))
			reset_vma_resv_huge_pages(tmp);

Then in alloc_huge_page() we checked vma_resv_map() mostly everywhere we'd
check avoid_reserve too (either in vma_needs_reservation, or calculating
deferred_reserve).  It seems to be mostly useful when vma_resv_map() existed.

But I completely agree I should pass in "1" here in v2.

Thanks,

-- 
Peter Xu