On Tue, 25 Sep 2012 17:13:27 +0900
Minchan Kim <minchan@xxxxxxxxxx> wrote:
> I see. To me, your saying is better than current comment.
> I hope comment could be more explicit.
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index df01b4e..f1d2cc7 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -542,8 +542,9 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> * splitting and collapsing (collapsing has already happened
> * if PageLRU is set) but the lock is not necessarily taken
> * here and it is wasteful to take it just to check transhuge.
> - * Check transhuge without lock and skip if it's either a
> - * transhuge or hugetlbfs page.
> + * Check transhuge without lock and *skip* if it's either a
> + * transhuge or hugetlbfs page because it's not safe to call
> + * compound_order.
> */
> if (PageTransHuge(page)) {
> if (!locked)
Going a bit further:
--- a/mm/compaction.c~mm-compaction-acquire-the-zone-lru_lock-as-late-as-possible-fix
+++ a/mm/compaction.c
@@ -415,7 +415,8 @@ isolate_migratepages_range(struct zone *
* if PageLRU is set) but the lock is not necessarily taken
* here and it is wasteful to take it just to check transhuge.
* Check transhuge without lock and skip if it's either a
- * transhuge or hugetlbfs page.
+ * transhuge or hugetlbfs page because calling compound_order()
+ * requires lru_lock to exclude isolation and splitting.
*/
if (PageTransHuge(page)) {
if (!locked)
_
but... the requirement to hold lru_lock for compound_order() is news
to me. It doesn't seem to be written down or explained anywhere, and
one wonders why the cheerily undocumented compound_lock() doesn't have
this effect. What's going on here??
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