[RFC PATCH 38/40] mm: compaction: skip empty tainted superpageblocks as migration source

[Rik van Riel <riel@surriel.com>]

Tainted superpageblocks with nr_movable == 0 hold only unmovable and/or
reclaimable pages.  None are migration candidates (compaction only moves
migratable pages), so scanning them is pure overhead.  Bail out at the
top of isolate_migratepages_block() when the current pageblock falls
inside such an SPB, returning 0 without performing any per-page scan.

The caller (isolate_migratepages) then advances to the next pageblock
and re-enters; each pageblock in the empty tainted SPB takes the same
fast bail, so the whole SPB is effectively skipped without per-page
scan work.  A tainted SPB with nr_movable > 0 still gets scanned
normally, preserving correctness.

Signed-off-by: Rik van Riel <riel@surriel.com>
Assisted-by: Claude:claude-opus-4.7 syzkaller
---
 mm/compaction.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index e4ba21072435..f9de52875c88 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -873,6 +873,27 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 
 	cc->migrate_pfn = low_pfn;
 
+	/*
+	 * Fast-path: a tainted SPB with no movable content has nothing to
+	 * migrate from this pageblock.  Bail out immediately and let the
+	 * caller move on to the next pageblock; the caller will re-enter
+	 * this function for each subsequent pageblock in the SPB and bail
+	 * the same way, so the whole SPB is effectively skipped without
+	 * any per-page scan work.
+	 */
+	if (cc->zone->nr_superpageblocks) {
+		struct superpageblock *sb =
+			pfn_to_superpageblock(cc->zone, low_pfn);
+
+		if (sb && spb_get_category(sb) == SB_TAINTED &&
+		    sb->nr_movable == 0) {
+			unsigned long sb_end =
+				ALIGN(low_pfn + 1, SUPERPAGEBLOCK_NR_PAGES);
+			cc->migrate_pfn = min(sb_end, end_pfn);
+			return 0;
+		}
+	}
+
 	/*
 	 * Ensure that there are not too many pages isolated from the LRU
 	 * list by either parallel reclaimers or compaction. If there are,
-- 
2.54.0