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