[RFC PATCH v2 2/5] filemap: add function filemap_map_folio_range()

[Yin Fengwei <fengwei.yin@intel.com>]

filemap_map_folio_range() maps partial/full folio. Comparing to
original filemap_map_pages(), it batched updates refcount and
get minor performance improvement for large folio.

a self cooked will-it-scale.page_fault3 like app (change file
write fault to read fault) with xfs filesystem got 2% performance
gain.

Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
---
 mm/filemap.c | 91 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 57 insertions(+), 34 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 992554c18f1f..9cc5edd8f998 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3351,6 +3351,56 @@ static inline struct folio *next_map_page(struct address_space *mapping,
 				  mapping, xas, end_pgoff);
 }
 
+/*
+ * Map sub-pages range [start_page, start_page + nr_pages) of folio.
+ * start_page is gotten from start by folio_page(folio, start)
+ */
+static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
+			struct folio *folio, unsigned long start,
+			unsigned long addr, unsigned int nr_pages)
+{
+	vm_fault_t ret = 0;
+	struct vm_area_struct *vma = vmf->vma;
+	struct file *file = vma->vm_file;
+	struct page *page = folio_page(folio, start);
+	unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
+	unsigned int ref_count = 0, count = 0;
+
+	do {
+		if (PageHWPoison(page))
+			continue;
+
+		if (mmap_miss > 0)
+			mmap_miss--;
+
+		/*
+		 * NOTE: If there're PTE markers, we'll leave them to be
+		 * handled in the specific fault path, and it'll prohibit the
+		 * fault-around logic.
+		 */
+		if (!pte_none(*vmf->pte))
+			continue;
+
+		if (vmf->address == addr)
+			ret = VM_FAULT_NOPAGE;
+
+		ref_count++;
+		do_set_pte(vmf, page, addr);
+		update_mmu_cache(vma, addr, vmf->pte);
+	} while (vmf->pte++, page++, addr += PAGE_SIZE, ++count < nr_pages);
+
+	/*
+	 * Restore the vmf->pte. Otherwise, it's possible vmf->pte point
+	 * to next page table entry if the last sub page in the range is
+	 * mapped to the last entry of page table.
+	 */
+	vmf->pte -= nr_pages;
+	folio_ref_add(folio, ref_count);
+	WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
+
+	return ret;
+}
+
 vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 			     pgoff_t start_pgoff, pgoff_t end_pgoff)
 {
@@ -3361,9 +3411,9 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	unsigned long addr;
 	XA_STATE(xas, &mapping->i_pages, start_pgoff);
 	struct folio *folio;
-	struct page *page;
 	unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
 	vm_fault_t ret = 0;
+	int nr_pages = 0;
 
 	rcu_read_lock();
 	folio = first_map_page(mapping, &xas, end_pgoff);
@@ -3378,45 +3428,18 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
 	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
 	do {
-again:
-		page = folio_file_page(folio, xas.xa_index);
-		if (PageHWPoison(page))
-			goto unlock;
-
-		if (mmap_miss > 0)
-			mmap_miss--;
+		unsigned long end;
 
 		addr += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
 		vmf->pte += xas.xa_index - last_pgoff;
 		last_pgoff = xas.xa_index;
+		end = folio->index + folio_nr_pages(folio) - 1;
+		nr_pages = min(end, end_pgoff) - xas.xa_index + 1;
 
-		/*
-		 * NOTE: If there're PTE markers, we'll leave them to be
-		 * handled in the specific fault path, and it'll prohibit the
-		 * fault-around logic.
-		 */
-		if (!pte_none(*vmf->pte))
-			goto unlock;
-
-		/* We're about to handle the fault */
-		if (vmf->address == addr)
-			ret = VM_FAULT_NOPAGE;
+		ret |=	filemap_map_folio_range(vmf, folio,
+				xas.xa_index - folio->index, addr, nr_pages);
+		xas.xa_index = end;
 
-		do_set_pte(vmf, page, addr);
-		/* no need to invalidate: a not-present page won't be cached */
-		update_mmu_cache(vma, addr, vmf->pte);
-		if (folio_more_pages(folio, xas.xa_index, end_pgoff)) {
-			xas.xa_index++;
-			folio_ref_inc(folio);
-			goto again;
-		}
-		folio_unlock(folio);
-		continue;
-unlock:
-		if (folio_more_pages(folio, xas.xa_index, end_pgoff)) {
-			xas.xa_index++;
-			goto again;
-		}
 		folio_unlock(folio);
 		folio_put(folio);
 	} while ((folio = next_map_page(mapping, &xas, end_pgoff)) != NULL);
-- 
2.30.2