[RFC PATCH v2 15/47] hugetlbfs: for unmapping, treat HGM-mapped pages as potentially mapped

[James Houghton <jthoughton@xxxxxxxxxx>]

hugetlb_vma_maps_page was mostly used as an optimization: if the VMA
isn't mapping a page, then we don't have to attempt to unmap it again.
We are still able to call the unmap routine if we need to.

For high-granularity mapped pages, we can't easily do a full walk to see
if the page is actually mapped or not, so simply return that it might
be.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
 fs/hugetlbfs/inode.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 7f836f8f9db1..a7ab62e39b8c 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -383,21 +383,34 @@ static void hugetlb_delete_from_page_cache(struct folio *folio)
  * mutex for the page in the mapping.  So, we can not race with page being
  * faulted into the vma.
  */
-static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
-				unsigned long addr, struct page *page)
+static bool hugetlb_vma_maybe_maps_page(struct vm_area_struct *vma,
+					unsigned long addr, struct page *page)
 {
 	pte_t *ptep, pte;
+	struct hugetlb_pte hpte;
+	struct hstate *h = hstate_vma(vma);
 
-	ptep = huge_pte_offset(vma->vm_mm, addr,
-			huge_page_size(hstate_vma(vma)));
+	ptep = huge_pte_offset(vma->vm_mm, addr, huge_page_size(h));
 
 	if (!ptep)
 		return false;
 
+	hugetlb_pte_populate(&hpte, ptep, huge_page_shift(h),
+			hpage_size_to_level(huge_page_size(h)));
+
 	pte = huge_ptep_get(ptep);
 	if (huge_pte_none(pte) || !pte_present(pte))
 		return false;
 
+	if (!hugetlb_pte_present_leaf(&hpte, pte))
+		/*
+		 * The top-level PTE is not a leaf, so it's possible that a PTE
+		 * under us is mapping the page. We aren't holding the VMA
+		 * lock, so it is unsafe to continue the walk further. Instead,
+		 * return true to indicate that we might be mapping the page.
+		 */
+		return true;
+
 	if (pte_page(pte) == page)
 		return true;
 
@@ -457,7 +470,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
 		v_start = vma_offset_start(vma, start);
 		v_end = vma_offset_end(vma, end);
 
-		if (!hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page))
+		if (!hugetlb_vma_maybe_maps_page(vma, vma->vm_start + v_start,
+					page))
 			continue;
 
 		if (!hugetlb_vma_trylock_write(vma)) {
@@ -507,7 +521,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
 		 */
 		v_start = vma_offset_start(vma, start);
 		v_end = vma_offset_end(vma, end);
-		if (hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page))
+		if (hugetlb_vma_maybe_maps_page(vma, vma->vm_start + v_start,
+					page))
 			unmap_hugepage_range(vma, vma->vm_start + v_start,
 						v_end, NULL,
 						ZAP_FLAG_DROP_MARKER);
-- 
2.38.0.135.g90850a2211-goog