[PATCH 3/9] mm/compaction: correctly return failure with bogus compound_order in strict mode

[Kemeng Shi]

In strict mode, we should return 0 if there is any hole in pageblock. If
we successfully isolated pages at beginning at pageblock and then have a
bogus compound_order outside pageblock in next page. We will abort search
loop with blockpfn > end_pfn. Although we will limit blockpfn to end_pfn,
we will treat it as a successful isolation in strict mode as blockpfn is
not < end_pfn and return partial isolated pages. Then
isolate_freepages_range may success unexpectly with hole in isolated
range.
This patch also removes unnecessary limit for blockpfn to go outside
by buddy page introduced in fixed commit or by stride introduced after
fixed commit. Caller could use returned blockpfn to check if full
pageblock is scanned by test if blockpfn >= end and to get next pfn to
scan inside isolate_freepages_block on demand.

Fixes: 9fcd6d2e052ee ("mm, compaction: skip compound pages by order in free scanner")
Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 mm/compaction.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index fa1b100b0d10..684f6e6cd8bc 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -631,6 +631,14 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 				page += (1UL << order) - 1;
 				nr_scanned += (1UL << order) - 1;
 			}
+			/*
+			 * There is a tiny chance that we have read bogus
+			 * compound_order(), so be careful to not go outside
+			 * of the pageblock.
+			 */
+			if (unlikely(blockpfn >= end_pfn))
+				blockpfn = end_pfn - 1;
+
 			goto isolate_fail;
 		}
 
@@ -677,17 +685,10 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 	if (locked)
 		spin_unlock_irqrestore(&cc->zone->lock, flags);
 
-	/*
-	 * There is a tiny chance that we have read bogus compound_order(),
-	 * so be careful to not go outside of the pageblock.
-	 */
-	if (unlikely(blockpfn > end_pfn))
-		blockpfn = end_pfn;
-
 	trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
 					nr_scanned, total_isolated);
 
-	/* Record how far we have got within the block */
+	/* Record how far we have got */
 	*start_pfn = blockpfn;
 
 	/*
@@ -1443,7 +1444,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn)
 	isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
 
 	/* Skip this pageblock in the future as it's full or nearly full */
-	if (start_pfn == end_pfn && !cc->no_set_skip_hint)
+	if (start_pfn >= end_pfn && !cc->no_set_skip_hint)
 		set_pageblock_skip(page);
 }
 
@@ -1712,7 +1713,7 @@ static void isolate_freepages(struct compact_control *cc)
 					block_end_pfn, freelist, stride, false);
 
 		/* Update the skip hint if the full pageblock was scanned */
-		if (isolate_start_pfn == block_end_pfn)
+		if (isolate_start_pfn >= block_end_pfn)
 			update_pageblock_skip(cc, page, block_start_pfn -
 					      pageblock_nr_pages);
 
-- 
2.30.0