[PATCH v3 12/25] mm/memory: Enhance insert_page_into_pte_locked() to create writable mappings

[Alistair Popple <apopple@xxxxxxxxxx>]

In preparation for using insert_page() for DAX, enhance
insert_page_into_pte_locked() to handle establishing writable
mappings.  Recall that DAX returns VM_FAULT_NOPAGE after installing a
PTE which bypasses the typical set_pte_range() in finish_fault.

Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>

---

Changes since v2:

 - New patch split out from "mm/memory: Add dax_insert_pfn"
---
 mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 24a34a4..323662c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2042,19 +2042,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
 }
 
 static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
-			unsigned long addr, struct page *page, pgprot_t prot)
+				unsigned long addr, struct page *page,
+				pgprot_t prot, bool mkwrite)
 {
 	struct folio *folio = page_folio(page);
+	pte_t entry = ptep_get(pte);
 	pte_t pteval;
 
-	if (!pte_none(ptep_get(pte)))
-		return -EBUSY;
+	if (!pte_none(entry)) {
+		if (!mkwrite)
+			return -EBUSY;
+
+		/*
+		 * For read faults on private mappings the PFN passed in may not
+		 * match the PFN we have mapped if the mapped PFN is a writeable
+		 * COW page.  In the mkwrite case we are creating a writable PTE
+		 * for a shared mapping and we expect the PFNs to match. If they
+		 * don't match, we are likely racing with block allocation and
+		 * mapping invalidation so just skip the update.
+		 */
+		if (pte_pfn(entry) != page_to_pfn(page)) {
+			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
+			return -EFAULT;
+		}
+		entry = maybe_mkwrite(entry, vma);
+		entry = pte_mkyoung(entry);
+		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
+			update_mmu_cache(vma, addr, pte);
+		return 0;
+	}
+
 	/* Ok, finally just insert the thing.. */
 	pteval = mk_pte(page, prot);
 	if (unlikely(is_zero_folio(folio))) {
 		pteval = pte_mkspecial(pteval);
 	} else {
 		folio_get(folio);
+		entry = mk_pte(page, prot);
+		if (mkwrite) {
+			entry = pte_mkyoung(entry);
+			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+		}
 		inc_mm_counter(vma->vm_mm, mm_counter_file(folio));
 		folio_add_file_rmap_pte(folio, page, vma);
 	}
@@ -2063,7 +2091,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
 }
 
 static int insert_page(struct vm_area_struct *vma, unsigned long addr,
-			struct page *page, pgprot_t prot)
+			struct page *page, pgprot_t prot, bool mkwrite)
 {
 	int retval;
 	pte_t *pte;
@@ -2076,7 +2104,8 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr,
 	pte = get_locked_pte(vma->vm_mm, addr, &ptl);
 	if (!pte)
 		goto out;
-	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot);
+	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot,
+					mkwrite);
 	pte_unmap_unlock(pte, ptl);
 out:
 	return retval;
@@ -2090,7 +2119,7 @@ static int insert_page_in_batch_locked(struct vm_area_struct *vma, pte_t *pte,
 	err = validate_page_before_insert(vma, page);
 	if (err)
 		return err;
-	return insert_page_into_pte_locked(vma, pte, addr, page, prot);
+	return insert_page_into_pte_locked(vma, pte, addr, page, prot, false);
 }
 
 /* insert_pages() amortizes the cost of spinlock operations
@@ -2226,7 +2255,7 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
 		BUG_ON(vma->vm_flags & VM_PFNMAP);
 		vm_flags_set(vma, VM_MIXEDMAP);
 	}
-	return insert_page(vma, addr, page, vma->vm_page_prot);
+	return insert_page(vma, addr, page, vma->vm_page_prot, false);
 }
 EXPORT_SYMBOL(vm_insert_page);
 
@@ -2506,7 +2535,7 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
 		 * result in pfn_t_has_page() == false.
 		 */
 		page = pfn_to_page(pfn_t_to_pfn(pfn));
-		err = insert_page(vma, addr, page, pgprot);
+		err = insert_page(vma, addr, page, pgprot, mkwrite);
 	} else {
 		return insert_pfn(vma, addr, pfn, pgprot, mkwrite);
 	}
-- 
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