Re: [Qemu-devel] [PATCH 5/9] mm: compaction: Acquire the zone->lru_lock as late as possible

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 25 Sep 2012 17:13:27 +0900
Minchan Kim <minchan@kernel.org> 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??