[PATCH v4 1/6] mm: memory: extend finish_fault() to support large folio

[Baolin Wang]

Add large folio mapping establishment support for finish_fault() as a
preparation, to support multi-size THP allocation of anonymous shmem pages
in the following patches.

Keep the same behavior (per-page fault) for non-anon shmem to avoid inflating
the RSS unintentionally, and we can discuss what size of mapping to build
when extending mTHP to control non-anon shmem in the future.

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
 mm/memory.c | 57 +++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 47 insertions(+), 10 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index eef4e482c0c2..1f7be4c6aac4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4831,9 +4831,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	struct page *page;
+	struct folio *folio;
 	vm_fault_t ret;
 	bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
 		      !(vma->vm_flags & VM_SHARED);
+	int type, nr_pages, i;
+	unsigned long addr = vmf->address;
 
 	/* Did we COW the page? */
 	if (is_cow)
@@ -4864,24 +4867,58 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 			return VM_FAULT_OOM;
 	}
 
+	folio = page_folio(page);
+	nr_pages = folio_nr_pages(folio);
+
+	/*
+	 * Using per-page fault to maintain the uffd semantics, and same
+	 * approach also applies to non-anonymous-shmem faults to avoid
+	 * inflating the RSS of the process.
+	 */
+	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
+		nr_pages = 1;
+	} else if (nr_pages > 1) {
+		pgoff_t idx = folio_page_idx(folio, page);
+		/* The page offset of vmf->address within the VMA. */
+		pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff;
+
+		/*
+		 * Fallback to per-page fault in case the folio size in page
+		 * cache beyond the VMA limits.
+		 */
+		if (unlikely(vma_off < idx ||
+			     vma_off + (nr_pages - idx) > vma_pages(vma))) {
+			nr_pages = 1;
+		} else {
+			/* Now we can set mappings for the whole large folio. */
+			addr = vmf->address - idx * PAGE_SIZE;
+			page = &folio->page;
+		}
+	}
+
 	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-				      vmf->address, &vmf->ptl);
+				       addr, &vmf->ptl);
 	if (!vmf->pte)
 		return VM_FAULT_NOPAGE;
 
 	/* Re-check under ptl */
-	if (likely(!vmf_pte_changed(vmf))) {
-		struct folio *folio = page_folio(page);
-		int type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
-
-		set_pte_range(vmf, folio, page, 1, vmf->address);
-		add_mm_counter(vma->vm_mm, type, 1);
-		ret = 0;
-	} else {
-		update_mmu_tlb(vma, vmf->address, vmf->pte);
+	if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) {
+		update_mmu_tlb(vma, addr, vmf->pte);
+		ret = VM_FAULT_NOPAGE;
+		goto unlock;
+	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
+		update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
 		ret = VM_FAULT_NOPAGE;
+		goto unlock;
 	}
 
+	folio_ref_add(folio, nr_pages - 1);
+	set_pte_range(vmf, folio, page, nr_pages, addr);
+	type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
+	add_mm_counter(vma->vm_mm, type, nr_pages);
+	ret = 0;
+
+unlock:
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
 	return ret;
 }
-- 
2.39.3