[PATCH 2/8] mm/khugepaged: factor out page cache folio reading

[Pedro Falcato]

The two mechanisms (shmem, filemap) are quite similar and only change in
subtleties. There is no need to have two variants. Factor this out into
its own separate function, shared by both paths.

Signed-off-by: Pedro Falcato <pfalcato@xxxxxxx>
---
 mm/khugepaged.c | 67 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 44 insertions(+), 23 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d4de507ac001..4cc6917a55c7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2230,42 +2230,63 @@ struct collapse_file_state {
 	unsigned int is_shmem : 1;
 };
 
+static struct folio *collapse_read_folio(pgoff_t index, struct collapse_file_state *state)
+{
+	const bool may_bring_uptodate = state->is_shmem;
+	struct folio *folio = state->folio;
+
+	if (!folio || xa_is_value(folio) ||
+	   (may_bring_uptodate && !folio_test_uptodate(folio))) {
+		xas_unlock_irq(state->xas);
+		if (state->is_shmem) {
+			/* swap in or instantiate fallocated page */
+			if (shmem_get_folio(state->mapping->host, index, 0,
+					    &folio, SGP_NOALLOC))
+				return NULL;
+		} else {
+			page_cache_sync_readahead(state->mapping, &state->file->f_ra,
+						  state->file, index,
+						  state->end - index);
+			folio = filemap_lock_folio(state->mapping, index);
+			if (IS_ERR(folio))
+				return NULL;
+		}
+	}
+
+	return folio;
+}
+
 static enum scan_result prepare_collapse_file_folio(pgoff_t index, struct collapse_file_state *state)
 {
 	struct address_space *mapping = state->mapping;
 	enum scan_result result = SCAN_SUCCEED;
 	const int is_shmem = state->is_shmem;
-	struct folio *folio = state->folio;
+	struct folio *folio;
+
+	folio = collapse_read_folio(index, state);
+	if (!folio)
+		return SCAN_FAIL;
+	if (folio != state->folio) {
+		/*
+		 * collapse_read_folio() got a new folio. This folio is locked
+		 * and ref'd up. Nothing tricky (dirty, writeback, etc) can
+		 * happen, so bail now. But before that, drain the local LRU
+		 * add batch. Otherwise, it's very possible folio_isolate_lru()
+		 * will not succeed.
+		 */
+		lru_add_drain();
+		goto xa_unlocked;
+	}
 
 	if (is_shmem) {
-		if (xa_is_value(folio) || !folio_test_uptodate(folio)) {
-			xas_unlock_irq(state->xas);
-			/* swap in or instantiate fallocated page */
-			if (shmem_get_folio(mapping->host, index, 0,
-					&folio, SGP_NOALLOC))
-				result = SCAN_FAIL;
-			/* drain lru cache to help folio_isolate_lru() */
-			lru_add_drain();
-			goto xa_unlocked;
-		} else if (folio_trylock(folio)) {
+		if (folio_trylock(folio)) {
 			folio_get(folio);
 		} else {
 			result = SCAN_PAGE_LOCK;
 			goto xa_locked;
 		}
 	} else {	/* !is_shmem */
-		if (!folio || xa_is_value(folio)) {
-			xas_unlock_irq(state->xas);
-			page_cache_sync_readahead(mapping, &state->file->f_ra,
-						  state->file, index,
-						  state->end - index);
-			/* drain lru cache to help folio_isolate_lru() */
-			lru_add_drain();
-			folio = filemap_lock_folio(mapping, index);
-			if (IS_ERR(folio))
-				result = SCAN_FAIL;
-			goto xa_unlocked;
-		} else if (folio_test_dirty(folio)) {
+		if (folio_test_dirty(folio)) {
 			/*
 			 * This page is dirty because it hasn't
 			 * been flushed since first write.
-- 
2.55.0