[RFC PATCH v3 4/8] mm/gup: break out follow_one_pte() helper

[Rik van Riel <riel@surriel.com>]

follow_page_pte() is 92 lines and does two separate things: work out
which page a PTE maps, if any, and commit to the page it found. The
first half reaches the second through five exit paths, two of which
unwind the PTE lock in different ways.

Split the resolve half into follow_one_pte(), which returns the page it
resolved, NULL when the PTE cannot be followed, or the errno the caller
must report. follow_page_pte() is left with one unlock and one exit.

no_page_table() can look up the page cache, so the case that needs it is
recorded and the call made after dropping the PTE lock, as before.

No functional changes intended.

Suggested-by: David Hildenbrand <david@kernel.org>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 mm/gup.c | 100 +++++++++++++++++++++++++++++++------------------------
 1 file changed, 56 insertions(+), 44 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index b755ceaac0f5..5af6a23285de 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -868,74 +868,86 @@ static long follow_page_pte_commit(struct vm_area_struct *vma,
 	return 0;
 }
 
-static long follow_page_pte(struct vm_area_struct *vma,
-		unsigned long address, pmd_t *pmd, unsigned int flags,
-		struct page **pages)
+/*
+ * Resolve one present PTE to the page it maps. Returns no page and no error
+ * when the PTE cannot be followed but the caller may fault it in, and a
+ * negative errno when the caller must report the failure.
+ */
+static long follow_one_pte(struct vm_area_struct *vma, unsigned long address,
+		pte_t *ptep, pte_t pte, unsigned int flags, struct page **pagep)
 {
-	struct mm_struct *mm = vma->vm_mm;
-	struct folio *folio;
 	struct page *page;
-	spinlock_t *ptl;
-	pte_t *ptep, pte;
-	long ret;
 
-	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
-	if (!ptep)
-		return no_page_table(vma, flags, address);
-	pte = ptep_get(ptep);
+	*pagep = NULL;
+
 	if (!pte_present(pte))
-		goto no_page;
+		return 0;
 	if (pte_protnone(pte) && !gup_can_follow_protnone(vma, flags))
-		goto no_page;
+		return 0;
 
 	page = vm_normal_page(vma, address, pte);
 
 	/*
 	 * We only care about anon pages in can_follow_write_pte().
 	 */
-	if ((flags & FOLL_WRITE) &&
-	    !can_follow_write_pte(pte, page, vma, flags)) {
-		ret = 0;
-		goto out;
-	}
+	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, page, vma, flags))
+		return 0;
 
 	if (unlikely(!page)) {
 		if (flags & FOLL_DUMP) {
 			/* Avoid special (like zero) pages in core dumps */
-			ret = -EFAULT;
-			goto out;
-		}
-
-		if (is_zero_pfn(pte_pfn(pte))) {
-			page = pte_page(pte);
-		} else {
-			ret = follow_pfn_pte(vma, address, ptep, flags);
-			goto out;
+			return -EFAULT;
 		}
+		if (!is_zero_pfn(pte_pfn(pte)))
+			return follow_pfn_pte(vma, address, ptep, flags);
+		page = pte_page(pte);
 	}
-	folio = page_folio(page);
 
-	if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) {
-		ret = -EMLINK;
-		goto out;
-	}
+	if (!pte_write(pte) && gup_must_unshare(vma, flags, page))
+		return -EMLINK;
 
 	VM_WARN_ON_ONCE_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
 			     !PageAnonExclusive(page), page);
 
-	ret = follow_page_pte_commit(vma, address, folio, page, pte, flags,
-				     pages);
-	if (ret)
-		goto out;
-	ret = 1;
-out:
+	*pagep = page;
+	return 0;
+}
+
+static long follow_page_pte(struct vm_area_struct *vma,
+		unsigned long address, pmd_t *pmd, unsigned int flags,
+		struct page **pages)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	bool need_no_page_table = false;
+	struct page *page;
+	spinlock_t *ptl;
+	pte_t *ptep, pte;
+	long ret;
+
+	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
+	if (!ptep)
+		return no_page_table(vma, flags, address);
+	pte = ptep_get(ptep);
+
+	ret = follow_one_pte(vma, address, ptep, pte, flags, &page);
+	if (!ret && page) {
+		ret = follow_page_pte_commit(vma, address, page_folio(page),
+					     page, pte, flags, pages);
+		if (!ret)
+			ret = 1;
+	} else if (!ret && pte_none(pte)) {
+		/*
+		 * no_page_table() may look up the page cache, so it cannot run
+		 * under the PTE lock.
+		 */
+		need_no_page_table = true;
+	}
+
 	pte_unmap_unlock(ptep, ptl);
+
+	if (need_no_page_table)
+		return no_page_table(vma, flags, address);
 	return ret;
-no_page:
-	pte_unmap_unlock(ptep, ptl);
-	if (!pte_none(pte))
-		return 0;
-	return no_page_table(vma, flags, address);
 }
 
 static long follow_pmd_mask(struct vm_area_struct *vma,
-- 
2.55.0