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    SubjectRe: [PATCH -mm v2] mm: have order > 0 compaction start off where it left
    On 06/28/2012 07:27 PM, Minchan Kim wrote:

    >> index 7ea259d..2668b77 100644
    >> --- a/mm/compaction.c
    >> +++ b/mm/compaction.c
    >> @@ -422,6 +422,17 @@ static void isolate_freepages(struct zone *zone,
    >> pfn -= pageblock_nr_pages) {
    >> unsigned long isolated;
    >>
    >> + /*
    >> + * Skip ahead if another thread is compacting in the area
    >> + * simultaneously. If we wrapped around, we can only skip
    >> + * ahead if zone->compact_cached_free_pfn also wrapped to
    >> + * above our starting point.
    >> + */
    >> + if (cc->order> 0&& (!cc->wrapped ||
    >
    >
    > So if (partial_compaction(cc)&& ... ) or if (!full_compaction(cc)&& ...

    I am not sure that we want to abstract away what is happening
    here. We also are quite explicit with the meaning of cc->order
    in compact_finished and other places in the compaction code.

    >> + zone->compact_cached_free_pfn>
    >> + cc->start_free_pfn))
    >> + pfn = min(pfn, zone->compact_cached_free_pfn);
    >
    >
    > The pfn can be where migrate_pfn below?
    > I mean we need this?
    >
    > if (pfn<= low_pfn)
    > goto out;

    That is a good point. I guess there is a small possibility that
    another compaction thread is below us with cc->free_pfn and
    cc->migrate_pfn, and we just inherited its cc->free_pfn via
    zone->compact_cached_free_pfn, bringing us to below our own
    cc->migrate_pfn.

    Given that this was already possible with parallel compaction
    in the past, I am not sure how important it is. It could result
    in wasting a little bit of CPU, but your fix for it looks easy
    enough.

    Mel, any downside to compaction bailing (well, wrapping around)
    a little earlier, like Minchan suggested?

    >> @@ -463,6 +474,8 @@ static void isolate_freepages(struct zone *zone,
    >> */
    >> if (isolated)
    >> high_pfn = max(high_pfn, pfn);
    >> + if (cc->order> 0)
    >> + zone->compact_cached_free_pfn = high_pfn;
    >
    >
    > Why do we cache high_pfn instead of pfn?

    Reading the code, because we may not have isolated every
    possible free page from this memory block. The same reason
    cc->free_pfn is set to high_pfn right before the function
    exits.

    > If we can't isolate any page, compact_cached_free_pfn would become low_pfn.
    > I expect it's not what you want.

    I guess we should only cache the value of high_pfn if
    we isolated some pages? In other words, this:

    if (isolated) {
    high_pfn = max(high_pfn, pfn);
    if (cc->order > 0)
    zone->compact_cached_free_pfn = high_pfn;
    }


    --
    All rights reversed


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