[PATCH v3 02/10] mm/memory: handle !page case in zap_present_pte() separately

[David Hildenbrand]

We don't need uptodate accessed/dirty bits, so in theory we could
replace ptep_get_and_clear_full() by an optimized ptep_clear_full()
function. Let's rely on the provided pte.

Further, there is no scenario where we would have to insert uffd-wp
markers when zapping something that is not a normal page (i.e., zeropage).
Add a sanity check to make sure this remains true.

should_zap_folio() no longer has to handle NULL pointers. This change
replaces 2/3 "!page/!folio" checks by a single "!page" one.

Note that arch_check_zapped_pte() on x86-64 checks the HW-dirty bit to
detect shadow stack entries. But for shadow stack entries, the HW dirty
bit (in combination with non-writable PTEs) is set by software. So for the
arch_check_zapped_pte() check, we don't have to sync against HW setting
the HW dirty bit concurrently, it is always set.

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/memory.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 5b0dc33133a6..4da6923709b2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1497,10 +1497,6 @@ static inline bool should_zap_folio(struct zap_details *details,
 	if (should_zap_cows(details))
 		return true;
 
-	/* E.g. the caller passes NULL for the case of a zero folio */
-	if (!folio)
-		return true;
-
 	/* Otherwise we should only zap non-anon folios */
 	return !folio_test_anon(folio);
 }
@@ -1538,24 +1534,28 @@ static inline void zap_present_pte(struct mmu_gather *tlb,
 		int *rss, bool *force_flush, bool *force_break)
 {
 	struct mm_struct *mm = tlb->mm;
-	struct folio *folio = NULL;
 	bool delay_rmap = false;
+	struct folio *folio;
 	struct page *page;
 
 	page = vm_normal_page(vma, addr, ptent);
-	if (page)
-		folio = page_folio(page);
+	if (!page) {
+		/* We don't need up-to-date accessed/dirty bits. */
+		ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
+		arch_check_zapped_pte(vma, ptent);
+		tlb_remove_tlb_entry(tlb, pte, addr);
+		VM_WARN_ON_ONCE(userfaultfd_wp(vma));
+		ksm_might_unmap_zero_page(mm, ptent);
+		return;
+	}
 
+	folio = page_folio(page);
 	if (unlikely(!should_zap_folio(details, folio)))
 		return;
 	ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
 	arch_check_zapped_pte(vma, ptent);
 	tlb_remove_tlb_entry(tlb, pte, addr);
 	zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
-	if (unlikely(!page)) {
-		ksm_might_unmap_zero_page(mm, ptent);
-		return;
-	}
 
 	if (!folio_test_anon(folio)) {
 		if (pte_dirty(ptent)) {
-- 
2.43.0