mm: khugepaged: use the number of pages in the folio to check the reference count

Use the number of pages in the folio to check the reference count as
preparation for supporting shmem mTHP collapse.

Link: https://lkml.kernel.org/r/9ea49262308de28957596cc6e8edc2d3a4f54659.1724140601.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 76f05215ee87ea9297ff6eac3ff6b9c5a30d9381..e015c94eba09d3e25fa3390028475f87196336fa 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1988,9 +1988,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
                VM_BUG_ON_FOLIO(folio != xa_load(xas.xa, index), folio);
 
                /*
-                * We control three references to the folio:
+                * We control 2 + nr_pages references to the folio:
                 *  - we hold a pin on it;
-                *  - one reference from page cache;
+                *  - nr_pages reference from page cache;
                 *  - one from lru_isolate_folio;
                 * If those are the only references, then any new usage
                 * of the folio will have to fetch it from the page
@@ -1998,7 +1998,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
                 * truncate, so any new usage will be blocked until we
                 * unlock folio after collapse/during rollback.
                 */
-               if (folio_ref_count(folio) != 3) {
+               if (folio_ref_count(folio) != 2 + folio_nr_pages(folio)) {
                        result = SCAN_PAGE_COUNT;
                        xas_unlock_irq(&xas);
                        folio_putback_lru(folio);
@@ -2181,7 +2181,7 @@ immap_locked:
                folio_clear_active(folio);
                folio_clear_unevictable(folio);
                folio_unlock(folio);
-               folio_put_refs(folio, 3);
+               folio_put_refs(folio, 2 + folio_nr_pages(folio));
        }
 
        goto out;