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


From: Andrew Morton
Subject: Re: [Qemu-devel] [PATCH 5/9] mm: compaction: Acquire the zone->lru_lock as late as possible
Date: Tue, 25 Sep 2012 14:39:31 -0700

On Tue, 25 Sep 2012 17:13:27 +0900
Minchan Kim <address@hidden> 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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