[PATCH v1 2/2] mm/huge_memory: don't mark refcounted pages special in vmf_insert_folio_pud()

[David Hildenbrand]

Marking PUDs that map a "normal" refcounted folios as special is
against our rules documented for vm_normal_page().

Fortunately, there are not that many pud_special() check that can be
mislead and are right now rather harmless: e.g., none so far
bases decisions whether to grab a folio reference on that decision.

Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
implications as it seems.

Getting this right will get more important as we introduce
folio_normal_page_pud() and start using it in more place where we
currently special-case based on other VMA flags.

Fix it by just inlining the relevant code, making the whole
pud_none() handling cleaner.

Add folio_mk_pud() to mimic what we do with folio_mk_pmd().

While at it, make sure that the pud that is non-none is actually present
before comparing PFNs.

Fixes: dbe54153296d ("mm/huge_memory: add vmf_insert_folio_pud()")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/mm.h | 15 +++++++++++++++
 mm/huge_memory.c   | 33 +++++++++++++++++++++++----------
 2 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0ef2ba0c667af..047c8261d4002 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1816,6 +1816,21 @@ static inline pmd_t folio_mk_pmd(struct folio *folio, pgprot_t pgprot)
 {
 	return pmd_mkhuge(pfn_pmd(folio_pfn(folio), pgprot));
 }
+
+/**
+ * folio_mk_pud - Create a PUD for this folio
+ * @folio: The folio to create a PUD for
+ * @pgprot: The page protection bits to use
+ *
+ * Create a page table entry for the first page of this folio.
+ * This is suitable for passing to set_pud_at().
+ *
+ * Return: A page table entry suitable for mapping this folio.
+ */
+static inline pud_t folio_mk_pud(struct folio *folio, pgprot_t pgprot)
+{
+	return pud_mkhuge(pfn_pud(folio_pfn(folio), pgprot));
+}
 #endif
 #endif /* CONFIG_MMU */
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f9e23dfea76f8..7b66a23089381 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1629,6 +1629,7 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
 	pud_t *pud = vmf->pud;
 	struct mm_struct *mm = vma->vm_mm;
 	spinlock_t *ptl;
+	pud_t entry;
 
 	if (addr < vma->vm_start || addr >= vma->vm_end)
 		return VM_FAULT_SIGBUS;
@@ -1637,20 +1638,32 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
 		return VM_FAULT_SIGBUS;
 
 	ptl = pud_lock(mm, pud);
-
-	/*
-	 * If there is already an entry present we assume the folio is
-	 * already mapped, hence no need to take another reference. We
-	 * still call insert_pfn_pud() though in case the mapping needs
-	 * upgrading to writeable.
-	 */
-	if (pud_none(*vmf->pud)) {
+	if (pud_none(*pud)) {
 		folio_get(folio);
 		folio_add_file_rmap_pud(folio, &folio->page, vma);
 		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
+
+		entry = folio_mk_pud(folio, vma->vm_page_prot);
+		if (write) {
+			entry = pud_mkyoung(pud_mkdirty(entry));
+			entry = maybe_pud_mkwrite(entry, vma);
+		}
+		set_pud_at(mm, addr, pud, entry);
+		update_mmu_cache_pud(vma, addr, pud);
+	} else if (pud_present(*pud) && write) {
+		/*
+		 * We only allow for upgrading write permissions if the
+		 * same folio is already mapped.
+		 */
+		if (pud_pfn(*pud) == folio_pfn(folio)) {
+			entry = pud_mkyoung(*pud);
+			entry = maybe_pud_mkwrite(pud_mkdirty(entry), vma);
+			if (pudp_set_access_flags(vma, addr, pud, entry, 1))
+				update_mmu_cache_pud(vma, addr, pud);
+		} else {
+			WARN_ON_ONCE(1);
+		}
 	}
-	insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)),
-		write);
 	spin_unlock(ptl);
 
 	return VM_FAULT_NOPAGE;
-- 
2.49.0