[PATCH v4 36/36] filemap: Batch PTE mappings

[Matthew Wilcox (Oracle)]

From: Yin Fengwei <fengwei.yin@xxxxxxxxx>

Call set_pte_range() once per contiguous range of the folio instead
of once per page.  This batches the updates to mm counters and the
rmap.

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>
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c | 36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index e2317623dcbf..7a1534460b55 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3483,11 +3483,12 @@ 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 count = 0;
+	pte_t *old_ptep = vmf->pte;
 
 	do {
-		if (PageHWPoison(page))
-			continue;
+		if (PageHWPoison(page + count))
+			goto skip;
 
 		if (mmap_miss > 0)
 			mmap_miss--;
@@ -3497,20 +3498,33 @@ 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(vmf->pte[count]))
+			goto skip;
 
 		if (vmf->address == addr)
 			ret = VM_FAULT_NOPAGE;
 
-		ref_count++;
-		set_pte_range(vmf, folio, page, 1, addr);
-	} while (vmf->pte++, page++, addr += PAGE_SIZE, ++count < nr_pages);
+		count++;
+		continue;
+skip:
+		if (count) {
+			set_pte_range(vmf, folio, page, count, addr);
+			folio_ref_add(folio, count);
+		}
 
-	/* Restore the vmf->pte */
-	vmf->pte -= nr_pages;
+		count++;
+		page += count;
+		vmf->pte += count;
+		addr += count * PAGE_SIZE;
+		count = 0;
+	} while (--nr_pages > 0);
+
+	if (count) {
+		set_pte_range(vmf, folio, page, count, addr);
+		folio_ref_add(folio, count);
+	}
 
-	folio_ref_add(folio, ref_count);
+	vmf->pte = old_ptep;
 	WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
 
 	return ret;
-- 
2.39.2