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SubjectRe: [patch 2/2] mm, compaction: persistently skip hugetlbfs pageblocks
On Wed, 23 Aug 2017, Vlastimil Babka wrote:

> > diff --git a/mm/compaction.c b/mm/compaction.c
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -217,6 +217,20 @@ static void reset_cached_positions(struct zone *zone)
> > pageblock_start_pfn(zone_end_pfn(zone) - 1);
> > }
> >
> > +/*
> > + * Hugetlbfs pages should consistenly be skipped until updated by the hugetlb
> > + * subsystem. It is always pointless to compact pages of pageblock_order and
> > + * the free scanner can reconsider when no longer huge.
> > + */
> > +static bool pageblock_skip_persistent(struct page *page, unsigned int order)
> > +{
> > + if (!PageHuge(page))
> > + return false;
> > + if (order != pageblock_order)
> > + return false;
> > + return true;
>
> Why just HugeTLBfs? There's also no point in migrating/finding free
> pages in THPs. Actually, any compound page of pageblock order?
>

Yes, any page where compound_order(page) == pageblock_order would probably
benefit from the same treatment. I haven't encountered such an issue,
however, so I thought it was best to restrict it only to hugetlb: hugetlb
memory usually sits in the hugetlb free pool and seldom gets freed under
normal conditions even when unmapped whereas thp is much more likely to be
unmapped and split. I wasn't sure that it was worth the pageblock skip.

> > +}
> > +
> > /*
> > * This function is called to clear all cached information on pageblocks that
> > * should be skipped for page isolation when the migrate and free page scanner
> > @@ -241,6 +255,8 @@ static void __reset_isolation_suitable(struct zone *zone)
> > continue;
> > if (zone != page_zone(page))
> > continue;
> > + if (pageblock_skip_persistent(page, compound_order(page)))
> > + continue;
>
> I like the idea of how persistency is achieved by rechecking in the reset.
>
> >
> > clear_pageblock_skip(page);
> > }
> > @@ -448,13 +464,15 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> > * and the only danger is skipping too much.
> > */
> > if (PageCompound(page)) {
> > - unsigned int comp_order = compound_order(page);
> > -
> > - if (likely(comp_order < MAX_ORDER)) {
> > - blockpfn += (1UL << comp_order) - 1;
> > - cursor += (1UL << comp_order) - 1;
> > + const unsigned int order = compound_order(page);
> > +
> > + if (pageblock_skip_persistent(page, order)) {
> > + set_pageblock_skip(page);
> > + blockpfn = end_pfn;
> > + } else if (likely(order < MAX_ORDER)) {
> > + blockpfn += (1UL << order) - 1;
> > + cursor += (1UL << order) - 1;
> > }
>
> Is this new code (and below) really necessary? The existing code should
> already lead to skip bit being set via update_pageblock_skip()?
>

I wanted to set the persistent pageblock skip regardless of
cc->ignore_skip_hint without a local change to update_pageblock_skip().

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