vmscan: move initialisation of mapping down

Now that we don't interrogate the BDI for congestion, we can delay looking
up the folio's mapping until we've got further through the function,
reducing register pressure and saving a call to folio_mapping for folios
we're adding to the swap cache.

Link: https://lkml.kernel.org/r/20220504182857.4013401-13-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 972557e..725a291 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1588,12 +1588,11 @@ retry:
                        stat->nr_unqueued_dirty += nr_pages;
 
                /*
-                * Treat this page as congested if the underlying BDI is or if
+                * Treat this page as congested if
                 * pages are cycling through the LRU so quickly that the
                 * pages marked for immediate reclaim are making it to the
                 * end of the LRU a second time.
                 */
-               mapping = page_mapping(page);
                if (writeback && PageReclaim(page))
                        stat->nr_congested += nr_pages;
 
@@ -1744,9 +1743,6 @@ retry:
                                        if (!add_to_swap(folio))
                                                goto activate_locked_split;
                                }
-
-                               /* Adding to swap updated mapping */
-                               mapping = page_mapping(page);
                        }
                } else if (PageSwapBacked(page) && PageTransHuge(page)) {
                        /* Split shmem THP */
@@ -1787,6 +1783,7 @@ retry:
                        }
                }
 
+               mapping = folio_mapping(folio);
                if (folio_test_dirty(folio)) {
                        /*
                         * Only kswapd can writeback filesystem folios