On 10/29/20 9:04 PM, Zi Yan wrote:
From: Zi Yan <ziy@xxxxxxxxxx>
In isolate_migratepages_block, when cc->alloc_contig is true, we are
able to isolate compound pages, nr_migratepages and nr_isolated did not
count compound pages correctly, causing us to isolate more pages than we
thought. Use thp_nr_pages to count pages. Otherwise, we might be trapped
in too_many_isolated while loop, since the actual isolated pages can go
up to COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,
since we stop isolation after cc->nr_migratepages reaches to
COMPACT_CLUSTER_MAX.
I wonder if a better fix would be to adjust the too_many_isolated() check so
that if we have non-zero cc->nr_migratepages, we bail out from further isolation
and migrate what we have immediately, instead of looping.
Because I can also imagine a hypothetical situation where multiple threads in
parallel cause too_many_isolated() to be true, and will all loop there forever.
The proposed fix should prevent such situation as well, AFAICT.
In addition, after we fix the issue above, cc->nr_migratepages could
never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
thus page isolation could not stop as we intended. Change the isolation
stop condition to >=.
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
mm/compaction.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index ee1f8439369e..0683a4999581 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
isolate_success:
list_add(&page->lru, &cc->migratepages);
- cc->nr_migratepages++;
- nr_isolated++;
+ cc->nr_migratepages += thp_nr_pages(page);
+ nr_isolated += thp_nr_pages(page);
/*
* Avoid isolating too much unless this block is being
@@ -1021,7 +1021,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
* or a lock is contended. For contention, isolate quickly to
* potentially remove one source of contention.
*/
- if (cc->nr_migratepages == COMPACT_CLUSTER_MAX &&
+ if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX &&
!cc->rescan && !cc->contended) {
++low_pfn;
break;
@@ -1132,7 +1132,7 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
if (!pfn)
break;
- if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
+ if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX)
break;
}
