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

[Mel Gorman]

On Tue, Sep 25, 2012 at 02:39:31PM -0700, Andrew Morton wrote:
> 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??
> 

The lru_lock is not *required* for compound_order(). Normally, users of
compound_order() know that the page is not going to collapse underneath
them. The slub allocator is not going to have a compound page it controls
disappear unexpectedly and does not need additional locking for example.

In the case where we are potentially dealing with a THP page, we have to
take into account if it can collapse underneath us. In this case, there is a
race between when when PageTransHuge is checked and compound_order is called.
The race is probably harmless but it was easy to take into account in
this case.

I think the comment saying that lru_lock is required is misleading. How
about this?

                 * Check TransHuge without lock and skip the whole
                 * pageblock if it's either a transhuge or hugetlbfs page
                 * as calling compound_order without preventing THP
                 * splitting the page underneath us may return
                 * surprising results.

-- 
Mel Gorman
SUSE Labs