mm: __isolate_lru_page_prepare() in isolate_migratepages_block()

__isolate_lru_page_prepare() conflates two unrelated functions, with the
flags to one disjoint from the flags to the other; and hides some of the
important checks outside of isolate_migratepages_block(), where the
sequence is better to be visible.  It comes from the days of lumpy
reclaim, before compaction, when the combination made more sense.

Move what's needed by mm/compaction.c isolate_migratepages_block() inline
there, and what's needed by mm/vmscan.c isolate_lru_pages() inline there.

Shorten "isolate_mode" to "mode", so the sequence of conditions is easier
to read.  Declare a "mapping" variable, to save one call to page_mapping()
(but not another: calling again after page is locked is necessary).
Simplify isolate_lru_pages() with a "move_to" list pointer.

Link: https://lkml.kernel.org/r/879d62a8-91cc-d3c6-fb3b-69768236df68@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Alex Shi <alexs@kernel.org>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 44 insertions, 7 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index b4e94cda30190..b72fb9da4c454 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -785,7 +785,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
  * @cc:		Compaction control structure.
  * @low_pfn:	The first PFN to isolate
  * @end_pfn:	The one-past-the-last PFN to isolate, within same pageblock
- * @isolate_mode: Isolation mode to be used.
+ * @mode:	Isolation mode to be used.
  *
  * Isolate all pages that can be migrated from the range specified by
  * [low_pfn, end_pfn). The range is expected to be within same pageblock.
@@ -798,7 +798,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
  */
 static int
 isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
-			unsigned long end_pfn, isolate_mode_t isolate_mode)
+			unsigned long end_pfn, isolate_mode_t mode)
 {
 	pg_data_t *pgdat = cc->zone->zone_pgdat;
 	unsigned long nr_scanned = 0, nr_isolated = 0;
@@ -806,6 +806,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 	unsigned long flags = 0;
 	struct lruvec *locked = NULL;
 	struct page *page = NULL, *valid_page = NULL;
+	struct address_space *mapping;
 	unsigned long start_pfn = low_pfn;
 	bool skip_on_failure = false;
 	unsigned long next_skip_pfn = 0;
@@ -990,7 +991,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 					locked = NULL;
 				}
 
-				if (!isolate_movable_page(page, isolate_mode))
+				if (!isolate_movable_page(page, mode))
 					goto isolate_success;
 			}
 
@@ -1002,15 +1003,15 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		 * so avoid taking lru_lock and isolating it unnecessarily in an
 		 * admittedly racy check.
 		 */
-		if (!page_mapping(page) &&
-		    page_count(page) > page_mapcount(page))
+		mapping = page_mapping(page);
+		if (!mapping && page_count(page) > page_mapcount(page))
 			goto isolate_fail;
 
 		/*
 		 * Only allow to migrate anonymous pages in GFP_NOFS context
 		 * because those do not depend on fs locks.
 		 */
-		if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
+		if (!(cc->gfp_mask & __GFP_FS) && mapping)
 			goto isolate_fail;
 
 		/*
@@ -1021,9 +1022,45 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (unlikely(!get_page_unless_zero(page)))
 			goto isolate_fail;
 
-		if (!__isolate_lru_page_prepare(page, isolate_mode))
+		/* Only take pages on LRU: a check now makes later tests safe */
+		if (!PageLRU(page))
+			goto isolate_fail_put;
+
+		/* Compaction might skip unevictable pages but CMA takes them */
+		if (!(mode & ISOLATE_UNEVICTABLE) && PageUnevictable(page))
+			goto isolate_fail_put;
+
+		/*
+		 * To minimise LRU disruption, the caller can indicate with
+		 * ISOLATE_ASYNC_MIGRATE that it only wants to isolate pages
+		 * it will be able to migrate without blocking - clean pages
+		 * for the most part.  PageWriteback would require blocking.
+		 */
+		if ((mode & ISOLATE_ASYNC_MIGRATE) && PageWriteback(page))
 			goto isolate_fail_put;
 
+		if ((mode & ISOLATE_ASYNC_MIGRATE) && PageDirty(page)) {
+			bool migrate_dirty;
+
+			/*
+			 * Only pages without mappings or that have a
+			 * ->migratepage callback are possible to migrate
+			 * without blocking. However, we can be racing with
+			 * truncation so it's necessary to lock the page
+			 * to stabilise the mapping as truncation holds
+			 * the page lock until after the page is removed
+			 * from the page cache.
+			 */
+			if (!trylock_page(page))
+				goto isolate_fail_put;
+
+			mapping = page_mapping(page);
+			migrate_dirty = !mapping || mapping->a_ops->migratepage;
+			unlock_page(page);
+			if (!migrate_dirty)
+				goto isolate_fail_put;
+		}
+
 		/* Try isolate the page */
 		if (!TestClearPageLRU(page))
 			goto isolate_fail_put;