Patchwork [5/9] mm: compaction: Acquire the zone->lru_lock as late as possible

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Submitter Andrew Morton
Date Sept. 25, 2012, 9:39 p.m.
Message ID <20120925143931.f404ca22.akpm@linux-foundation.org>
Download mbox | patch
Permalink /patch/186897/
State New
Headers show

Comments

Andrew Morton - Sept. 25, 2012, 9:39 p.m.
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:
Minchan Kim - Sept. 26, 2012, 12:23 a.m.
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@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??

First of all, I don't know why we should mention hugetlbfs in comment.
I don't know hugetlbfs well so I had a time to look through code but
can't find a place setting PG_lru so I'm not sure hugetlbfs page can
reach on this code. Please correct me if I was wrong.

On THP, I think compound_lock you mentioned is okay, too but I think
it's sort of optimization because we don't need both lru_lock and
compound_lock. If we hold lru_lock, we can't prevent race with
__split_huge_page_refcount so that the page couldn't be freed.

Namely, it's safe to call compound_order.

> 
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Mel Gorman - Sept. 26, 2012, 10:17 a.m.
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@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??
> 

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.

Patch

--- 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)