[RFC PATCH v4 4/4] filemap: batched update mm counter,rmap when map file folio

[Yin Fengwei <fengwei.yin@xxxxxxxxx>]

Use do_set_pte_range() in filemap_map_folio_range(). Which
batched updates mm counter, rmap for large folio.

With a will-it-scale.page_fault3 like app (change file write
fault testing to read fault testing. Trying to upstream it to
will-it-scale at [1]) got 15% performance gain on a 48C/96T
Cascade Lake test box with 96 processes running against xfs.

Perf data collected before/after the change:
  18.73%--page_add_file_rmap
          |
           --11.60%--__mod_lruvec_page_state
                     |
                     |--7.40%--__mod_memcg_lruvec_state
                     |          |
                     |           --5.58%--cgroup_rstat_updated
                     |
                      --2.53%--__mod_lruvec_state
                                |
                                 --1.48%--__mod_node_page_state

  9.93%--page_add_file_rmap_range
         |
          --2.67%--__mod_lruvec_page_state
                    |
                    |--1.95%--__mod_memcg_lruvec_state
                    |          |
                    |           --1.57%--cgroup_rstat_updated
                    |
                     --0.61%--__mod_lruvec_state
                               |
                                --0.54%--__mod_node_page_state

The running time of __mode_lruvec_page_state() is reduced about 9%.

[1]: https://github.com/antonblanchard/will-it-scale/pull/37

Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
---
 mm/filemap.c | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6f110b9e5d27..4452361e8858 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3354,11 +3354,12 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 	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;
+	unsigned int mapped = 0;
+	pte_t *pte = vmf->pte;
 
 	do {
 		if (PageHWPoison(page))
-			continue;
+			goto map;
 
 		if (mmap_miss > 0)
 			mmap_miss--;
@@ -3368,20 +3369,34 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 		 * handled in the specific fault path, and it'll prohibit the
 		 * fault-around logic.
 		 */
-		if (!pte_none(*vmf->pte))
-			continue;
+		if (!pte_none(pte[mapped]))
+			goto map;
 
 		if (vmf->address == addr)
 			ret = VM_FAULT_NOPAGE;
 
-		ref_count++;
-		do_set_pte(vmf, page, addr);
-	} while (vmf->pte++, page++, addr += PAGE_SIZE, ++count < nr_pages);
+		mapped++;
+		continue;
 
-	/* Restore the vmf->pte */
-	vmf->pte -= nr_pages;
+map:
+		if (mapped) {
+			do_set_pte_range(vmf, folio, addr, pte, start, mapped);
+			folio_ref_add(folio, mapped);
+		}
+
+		/* advance 1 to jump over the HWPoison or !pte_none entry */
+		mapped++;
+		start += mapped;
+		pte += mapped;
+		addr += mapped * PAGE_SIZE;
+		mapped = 0;
+	} while (page++, --nr_pages > 0);
+
+	if (mapped) {
+		do_set_pte_range(vmf, folio, addr, pte, start, mapped);
+		folio_ref_add(folio, mapped);
+	}
 
-	folio_ref_add(folio, ref_count);
 	WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
 
 	return ret;
-- 
2.30.2