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Subject[patch for-4.7] mm, compaction: prevent VM_BUG_ON when terminating freeing scanner
It's possible to isolate some freepages in a pageblock and then fail 
split_free_page() due to the low watermark check. In this case, we hit
VM_BUG_ON() because the freeing scanner terminated early without a
contended lock or enough freepages.

This should never have been a VM_BUG_ON() since it's not a fatal
condition. It should have been a VM_WARN_ON() at best, or even handled
gracefully.

Regardless, we need to terminate anytime the full pageblock scan was not
done. The logic belongs in isolate_freepages_block(), so handle its state
gracefully by terminating the pageblock loop and making a note to restart
at the same pageblock next time since it was not possible to complete the
scan this time.

Reported-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: David Rientjes <rientjes@google.com>
---
Note: I really dislike the low watermark check in split_free_page() and
consider it poor software engineering. The function should split a free
page, nothing more. Terminating memory compaction because of a low
watermark check when we're simply trying to migrate memory seems like an
arbitrary heuristic. There was an objection to removing it in the first
proposed patch, but I think we should really consider removing that
check so this is simpler.

mm/compaction.c | 37 +++++++++++++++----------------------
1 file changed, 15 insertions(+), 22 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1009,8 +1009,6 @@ static void isolate_freepages(struct compact_control *cc)
block_end_pfn = block_start_pfn,
block_start_pfn -= pageblock_nr_pages,
isolate_start_pfn = block_start_pfn) {
- unsigned long isolated;
-
/*
* This can iterate a massively long zone without finding any
* suitable migration targets, so periodically check if we need
@@ -1034,36 +1032,31 @@ static void isolate_freepages(struct compact_control *cc)
continue;

/* Found a block suitable for isolating free pages from. */
- isolated = isolate_freepages_block(cc, &isolate_start_pfn,
- block_end_pfn, freelist, false);
- /* If isolation failed early, do not continue needlessly */
- if (!isolated && isolate_start_pfn < block_end_pfn &&
- cc->nr_migratepages > cc->nr_freepages)
- break;
+ isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
+ freelist, false);

/*
- * If we isolated enough freepages, or aborted due to async
- * compaction being contended, terminate the loop.
- * Remember where the free scanner should restart next time,
- * which is where isolate_freepages_block() left off.
- * But if it scanned the whole pageblock, isolate_start_pfn
- * now points at block_end_pfn, which is the start of the next
- * pageblock.
- * In that case we will however want to restart at the start
- * of the previous pageblock.
+ * If we isolated enough freepages, or aborted due to lock
+ * contention, terminate.
*/
if ((cc->nr_freepages >= cc->nr_migratepages)
|| cc->contended) {
- if (isolate_start_pfn >= block_end_pfn)
+ if (isolate_start_pfn >= block_end_pfn) {
+ /*
+ * Restart at previous pageblock if more
+ * freepages can be isolated next time.
+ */
isolate_start_pfn =
block_start_pfn - pageblock_nr_pages;
+ }
break;
- } else {
+ } else if (isolate_start_pfn < block_end_pfn) {
/*
- * isolate_freepages_block() should not terminate
- * prematurely unless contended, or isolated enough
+ * If isolation failed early, do not continue
+ * needlessly.
*/
- VM_BUG_ON(isolate_start_pfn < block_end_pfn);
+ isolate_start_pfn = block_start_pfn;
+ break;
}
}

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