[PATCH v2 2/3] mm/huge_memory: don't mark refcounted folios special in vmf_insert_folio_pmd()

[David Hildenbrand <david@redhat.com>]

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

Fortunately, there are not that many pmd_special() check that can be
mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
would get this wrong right now are 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 use
folio_normal_page_pmd() in more places.

Fix it by teaching insert_pfn_pmd() to properly handle folios and
pfns -- moving refcount/mapcount/etc handling in there, renaming it to
insert_pmd(), and distinguishing between both cases using a new simple
"struct folio_or_pfn" structure.

Use folio_mk_pmd() to create a pmd for a folio cleanly.

Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/huge_memory.c | 58 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 39 insertions(+), 19 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 49b98082c5401..7e3e9028873e5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1372,9 +1372,17 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 	return __do_huge_pmd_anonymous_page(vmf);
 }
 
-static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
-		pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write,
-		pgtable_t pgtable)
+struct folio_or_pfn {
+	union {
+		struct folio *folio;
+		pfn_t pfn;
+	};
+	bool is_folio;
+};
+
+static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
+		pmd_t *pmd, struct folio_or_pfn fop, pgprot_t prot,
+		bool write, pgtable_t pgtable)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	pmd_t entry;
@@ -1382,8 +1390,11 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
 	lockdep_assert_held(pmd_lockptr(mm, pmd));
 
 	if (!pmd_none(*pmd)) {
+		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
+					  pfn_t_to_pfn(fop.pfn);
+
 		if (write) {
-			if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) {
+			if (pmd_pfn(*pmd) != pfn) {
 				WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
 				return -EEXIST;
 			}
@@ -1396,11 +1407,19 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
 		return -EEXIST;
 	}
 
-	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
-	if (pfn_t_devmap(pfn))
-		entry = pmd_mkdevmap(entry);
-	else
-		entry = pmd_mkspecial(entry);
+	if (fop.is_folio) {
+		entry = folio_mk_pmd(fop.folio, vma->vm_page_prot);
+
+		folio_get(fop.folio);
+		folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
+		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
+	} else {
+		entry = pmd_mkhuge(pfn_t_pmd(fop.pfn, prot));
+		if (pfn_t_devmap(fop.pfn))
+			entry = pmd_mkdevmap(entry);
+		else
+			entry = pmd_mkspecial(entry);
+	}
 	if (write) {
 		entry = pmd_mkyoung(pmd_mkdirty(entry));
 		entry = maybe_pmd_mkwrite(entry, vma);
@@ -1431,6 +1450,9 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
 	unsigned long addr = vmf->address & PMD_MASK;
 	struct vm_area_struct *vma = vmf->vma;
 	pgprot_t pgprot = vma->vm_page_prot;
+	struct folio_or_pfn fop = {
+		.pfn = pfn,
+	};
 	pgtable_t pgtable = NULL;
 	spinlock_t *ptl;
 	int error;
@@ -1458,8 +1480,8 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
 	pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
 
 	ptl = pmd_lock(vma->vm_mm, vmf->pmd);
-	error = insert_pfn_pmd(vma, addr, vmf->pmd, pfn, pgprot, write,
-			pgtable);
+	error = insert_pmd(vma, addr, vmf->pmd, fop, pgprot, write,
+			   pgtable);
 	spin_unlock(ptl);
 	if (error && pgtable)
 		pte_free(vma->vm_mm, pgtable);
@@ -1474,6 +1496,10 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio,
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long addr = vmf->address & PMD_MASK;
 	struct mm_struct *mm = vma->vm_mm;
+	struct folio_or_pfn fop = {
+		.folio = folio,
+		.is_folio = true,
+	};
 	spinlock_t *ptl;
 	pgtable_t pgtable = NULL;
 	int error;
@@ -1491,14 +1517,8 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio,
 	}
 
 	ptl = pmd_lock(mm, vmf->pmd);
-	if (pmd_none(*vmf->pmd)) {
-		folio_get(folio);
-		folio_add_file_rmap_pmd(folio, &folio->page, vma);
-		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
-	}
-	error = insert_pfn_pmd(vma, addr, vmf->pmd,
-			pfn_to_pfn_t(folio_pfn(folio)), vma->vm_page_prot,
-			write, pgtable);
+	error = insert_pmd(vma, addr, vmf->pmd, fop, vma->vm_page_prot,
+			   write, pgtable);
 	spin_unlock(ptl);
 	if (error && pgtable)
 		pte_free(mm, pgtable);
-- 
2.49.0