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