Re: [PATCH v3 1/4] mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()

On 17/07/2023 14:00, David Hildenbrand wrote:

On 14.07.23 18:17, Ryan Roberts wrote:

quoted

In preparation for FLEXIBLE_THP support, improve
folio_add_new_anon_rmap() to allow a non-pmd-mappable, large folio to be
passed to it. In this case, all contained pages are accounted using the
order-0 folio (or base page) scheme.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Yu Zhao <redacted>
Reviewed-by: Yin Fengwei <redacted>
---
  mm/rmap.c | 28 +++++++++++++++++++++-------
  1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 0c0d8857dfce..f293d072368a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c

@@ -1278,31 +1278,45 @@ void page_add_anon_rmap(struct page *page, struct

vm_area_struct *vma,
   * This means the inc-and-test can be bypassed.
   * The folio does not have to be locked.
   *
- * If the folio is large, it is accounted as a THP.  As the folio
+ * If the folio is pmd-mappable, it is accounted as a THP.  As the folio
   * is new, it's assumed to be mapped exclusively by a single process.
   */
  void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
          unsigned long address)
  {
-    int nr;
+    int nr = folio_nr_pages(folio);

-    VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
+    VM_BUG_ON_VMA(address < vma->vm_start ||
+            address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
      __folio_set_swapbacked(folio);

-    if (likely(!folio_test_pmd_mappable(folio))) {
+    if (!folio_test_large(folio)) {

Why remove the "likely" here? The patch itself does not change anything about
that condition.

Good question; I'm not sure why. Will have to put it down to bad copy/paste
fixup. Will put it back in the next version.

quoted

          /* increment count (starts at -1) */
          atomic_set(&folio->_mapcount, 0);
-        nr = 1;
+        __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
+    } else if (!folio_test_pmd_mappable(folio)) {
+        int i;
+
+        for (i = 0; i < nr; i++) {
+            struct page *page = folio_page(folio, i);
+
+            /* increment count (starts at -1) */
+            atomic_set(&page->_mapcount, 0);
+            __page_set_anon_rmap(folio, page, vma,
+                    address + (i << PAGE_SHIFT), 1);
+        }
+
+        /* increment count (starts at 0) */

That comment is a bit misleading. We're not talking about a mapcount as in the
other cases here.

Correct, I'm talking about _nr_pages_mapped, which starts 0, not -1 like
_mapcount. The comment was intended to be in the style used in other similar
places in rmap.c. I could change it to: "_nr_pages_mapped is 0-based, so set it
to the number of pages in the folio" or remove it entirely? What do you prefer?

quoted

+        atomic_set(&folio->_nr_pages_mapped, nr);
      } else {
          /* increment count (starts at -1) */
          atomic_set(&folio->_entire_mapcount, 0);
          atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);
-        nr = folio_nr_pages(folio);
+        __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
          __lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
      }

Apart from that, LGTM.

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