Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner

David Rientjes
2014-06-05T21:30:56Z

On Thu, 5 Jun 2014, Vlastimil Babka wrote:

> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index ae7db5f..3dce5a7 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -640,11 +640,18 @@ isolate_migratepages_range(struct zone *zone, struct
> > > compact_control *cc,
> > > }
> > >
> > > /*
> > > - * Skip if free. page_order cannot be used without zone->lock
> > > - * as nothing prevents parallel allocations or buddy merging.
> > > + * Skip if free. We read page order here without zone lock
> > > + * which is generally unsafe, but the race window is small and
> > > + * the worst thing that can happen is that we skip some
> > > + * potential isolation targets.
> >
> > Should we only be doing the low_pfn adjustment based on the order for
> > MIGRATE_ASYNC? It seems like sync compaction, including compaction that
> > is triggered from the command line, would prefer to scan over the
> > following pages.
>
> I thought even sync compaction would benefit from the skipped iterations. I'd
> say the probability of this race is smaller than probability of somebody
> allocating what compaction just freed.
>

Ok.

> > > diff --git a/mm/internal.h b/mm/internal.h
> > > index 1a8a0d4..6aa1f74 100644
> > > --- a/mm/internal.h
> > > +++ b/mm/internal.h
> > > @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct
> > > compact_control *cc,
> > > * general, page_zone(page)->lock must be held by the caller to prevent
> > > the
> > > * page from being allocated in parallel and returning garbage as the
> > > order.
> > > * If a caller does not hold page_zone(page)->lock, it must guarantee
> > > that the
> > > - * page cannot be allocated or merged in parallel.
> > > + * page cannot be allocated or merged in parallel. Alternatively, it must
> > > + * handle invalid values gracefully, and use page_order_unsafe() below.
> > > */
> > > static inline unsigned long page_order(struct page *page)
> > > {
> > > @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page
> > > *page)
> > > return page_private(page);
> > > }
> > >
> > > +/*
> > > + * Like page_order(), but for callers who cannot afford to hold the zone
> > > lock,
> > > + * and handle invalid values gracefully. ACCESS_ONCE is used so that if
> > > the
> > > + * caller assigns the result into a local variable and e.g. tests it for
> > > valid
> > > + * range before using, the compiler cannot decide to remove the variable
> > > and
> > > + * inline the function multiple times, potentially observing different
> > > values
> > > + * in the tests and the actual use of the result.
> > > + */
> > > +static inline unsigned long page_order_unsafe(struct page *page)
> > > +{
> > > + /*
> > > + * PageBuddy() should be checked by the caller to minimize race
> > > window,
> > > + * and invalid values must be handled gracefully.
> > > + */
> > > + return ACCESS_ONCE(page_private(page));
> > > +}
> > > +
> > > /* mm/util.c */
> > > void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
> > > struct vm_area_struct *prev, struct rb_node *rb_parent);
> >
> > I don't like this change at all, I don't think we should have header
> > functions that imply the context in which the function will be called. I
> > think it would make much more sense to just do
> > ACCESS_ONCE(page_order(page)) in the migration scanner with a comment.
>
> But that won't compile. It would have to be converted to a #define, unless
> there's some trick I don't know. Sure I would hope this could be done cleaner
> somehow.
>

Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner
with a comment about it being racy. It also helps to understand why
you're testing for order < MAX_ORDER before skipping low_pfn there which
is a little subtle right now.