[RFC PATCH V1] mm: remove update_mmu_cache() when page is zero page

[Xin Hao <xhao@xxxxxxxxxxxxxxxxx>]

The zero page is never written to, so it will never has any dirty cache
lines, and therefore there no need to be flushed.

Signed-off-by: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
---
 mm/huge_memory.c | 1 -
 mm/memory.c      | 5 ++++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8a7c1b344abe..679eb425e54f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -821,7 +821,6 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 			} else {
 				set_huge_zero_page(pgtable, vma->vm_mm, vma,
 						   haddr, vmf->pmd, zero_page);
-				update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
 				spin_unlock(vmf->ptl);
 			}
 		} else {
diff --git a/mm/memory.c b/mm/memory.c
index 4ba73f5aa8bb..3650e7cae26f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4034,6 +4034,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	struct page *page;
 	vm_fault_t ret = 0;
 	pte_t entry;
+	bool is_zero_page = false;

 	/* File mapping without ->vm_ops ? */
 	if (vma->vm_flags & VM_SHARED)
@@ -4075,6 +4076,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 			pte_unmap_unlock(vmf->pte, vmf->ptl);
 			return handle_userfault(vmf, VM_UFFD_MISSING);
 		}
+		is_zero_page = true;
 		goto setpte;
 	}

@@ -4126,7 +4128,8 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);

 	/* No need to invalidate - it was non-present before */
-	update_mmu_cache(vma, vmf->address, vmf->pte);
+	if (!is_zero_page)
+		update_mmu_cache(vma, vmf->address, vmf->pte);
 unlock:
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
 	return ret;
--
2.31.0