mm: split out a writeout helper from pageout

Patch series "stop passing a writeback_control to swap/shmem writeout",
v3.

This series was intended to remove the last remaining users of
AOP_WRITEPAGE_ACTIVATE after my other pending patches removed the rest,
but spectacularly failed at that.

But instead it nicely improves the code, and removes two pointers from
struct writeback_control.


This patch (of 6):

Move the code to write back swap / shmem folios into a self-contained
helper to keep prepare for refactoring it.

Link: https://lkml.kernel.org/r/20250610054959.2057526-1-hch@lst.de
Link: https://lkml.kernel.org/r/20250610054959.2057526-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Baolin Wang <baolin.wang@linux.aibaba.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 48 insertions, 46 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ad47244179bb..b6dd1708fe82 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -652,14 +652,55 @@ typedef enum {
 	PAGE_CLEAN,
 } pageout_t;
 
+static pageout_t writeout(struct folio *folio, struct address_space *mapping,
+		struct swap_iocb **plug, struct list_head *folio_list)
+{
+	struct writeback_control wbc = {
+		.sync_mode = WB_SYNC_NONE,
+		.nr_to_write = SWAP_CLUSTER_MAX,
+		.range_start = 0,
+		.range_end = LLONG_MAX,
+		.for_reclaim = 1,
+		.swap_plug = plug,
+	};
+	int res;
+
+	folio_set_reclaim(folio);
+
+	/*
+	 * The large shmem folio can be split if CONFIG_THP_SWAP is not enabled
+	 * or we failed to allocate contiguous swap entries.
+	 */
+	if (shmem_mapping(mapping)) {
+		if (folio_test_large(folio))
+			wbc.list = folio_list;
+		res = shmem_writeout(folio, &wbc);
+	} else {
+		res = swap_writeout(folio, &wbc);
+	}
+
+	if (res < 0)
+		handle_write_error(mapping, folio, res);
+	if (res == AOP_WRITEPAGE_ACTIVATE) {
+		folio_clear_reclaim(folio);
+		return PAGE_ACTIVATE;
+	}
+
+	/* synchronous write? */
+	if (!folio_test_writeback(folio))
+		folio_clear_reclaim(folio);
+
+	trace_mm_vmscan_write_folio(folio);
+	node_stat_add_folio(folio, NR_VMSCAN_WRITE);
+	return PAGE_SUCCESS;
+}
+
 /*
  * pageout is called by shrink_folio_list() for each dirty folio.
  */
 static pageout_t pageout(struct folio *folio, struct address_space *mapping,
 			 struct swap_iocb **plug, struct list_head *folio_list)
 {
-	int (*writeout)(struct folio *, struct writeback_control *);
-
 	/*
 	 * We no longer attempt to writeback filesystem folios here, other
 	 * than tmpfs/shmem.  That's taken care of in page-writeback.
@@ -690,51 +731,12 @@ static pageout_t pageout(struct folio *folio, struct address_space *mapping,
 		}
 		return PAGE_KEEP;
 	}
-	if (shmem_mapping(mapping))
-		writeout = shmem_writeout;
-	else if (folio_test_anon(folio))
-		writeout = swap_writeout;
-	else
-		return PAGE_ACTIVATE;
-
-	if (folio_clear_dirty_for_io(folio)) {
-		int res;
-		struct writeback_control wbc = {
-			.sync_mode = WB_SYNC_NONE,
-			.nr_to_write = SWAP_CLUSTER_MAX,
-			.range_start = 0,
-			.range_end = LLONG_MAX,
-			.for_reclaim = 1,
-			.swap_plug = plug,
-		};
 
-		/*
-		 * The large shmem folio can be split if CONFIG_THP_SWAP is
-		 * not enabled or contiguous swap entries are failed to
-		 * allocate.
-		 */
-		if (shmem_mapping(mapping) && folio_test_large(folio))
-			wbc.list = folio_list;
-
-		folio_set_reclaim(folio);
-		res = writeout(folio, &wbc);
-		if (res < 0)
-			handle_write_error(mapping, folio, res);
-		if (res == AOP_WRITEPAGE_ACTIVATE) {
-			folio_clear_reclaim(folio);
-			return PAGE_ACTIVATE;
-		}
-
-		if (!folio_test_writeback(folio)) {
-			/* synchronous write? */
-			folio_clear_reclaim(folio);
-		}
-		trace_mm_vmscan_write_folio(folio);
-		node_stat_add_folio(folio, NR_VMSCAN_WRITE);
-		return PAGE_SUCCESS;
-	}
-
-	return PAGE_CLEAN;
+	if (!shmem_mapping(mapping) && !folio_test_anon(folio))
+		return PAGE_ACTIVATE;
+	if (!folio_clear_dirty_for_io(folio))
+		return PAGE_CLEAN;
+	return writeout(folio, mapping, plug, folio_list);
 }
 
 /*