Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it left

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 3 Jul 2012 11:10:24 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> > >>>>>+          if (cc->order>   0)
> > >>>>>+                  zone->compact_cached_free_pfn = high_pfn;
> > >>>>
> > >>>>Is high_pfn guaranteed to be aligned to pageblock_nr_pages here?  I
> > >>>>assume so, if lots of code in other places is correct but it's
> > >>>>unobvious from reading this function.
> > >>>
> > >>>Reading the code a few more times, I believe that it is
> > >>>indeed aligned to pageblock size.
> > >>
> > >>I'll slip this into -next for a while.
> > >>
> > >>--- a/mm/compaction.c~isolate_freepages-check-that-high_pfn-is-aligned-as-expected
> > >>+++ a/mm/compaction.c
> > >>@@ -456,6 +456,7 @@ static void isolate_freepages(struct zon
> > >>                 }
> > >>                 spin_unlock_irqrestore(&zone->lock, flags);
> > >>
> > >>+               WARN_ON_ONCE(high_pfn&  (pageblock_nr_pages - 1));
> > >>                 /*
> > >>                  * Record the highest PFN we isolated pages from. When next
> > >>                  * looking for free pages, the search will restart here as
> > >
> > >I've triggered the following with today's -next:
> > 
> > I've been staring at the migrate code for most of the afternoon,
> > and am not sure how this is triggered.
> > 
> 
> That warning is placed in isolate_freepages(). When the migration
> scanner and free scanner have almost met it is possible for high_pfn to
> be
> 
> cc->migrate_pfn + pageblock_nr_pages
> 
> and that is not necessarily pageblock aligned. Forcing it to be aligned
> raises the possibility that the free scanner moves to another zone. This
> is very unlikely but could happen if a high zone was very small.
> 
> I should have caught this when the warning was proposed :( IMO it's
> safe to just drop the warning.

The rest of this patch takes care to ensure that
->compact_cached_free_pfn is aligned to pageblock_nr_pages.  But it now
appears that this particular site will violate that.

What's up?  Do we need to fix this site, or do we remove all that
make-compact_cached_free_pfn-aligned code?

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