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SubjectRe: [PATCH 23/25] mm, compaction: Be selective about what pageblocks to clear skip hints
On Fri, Jan 18, 2019 at 01:55:24PM +0100, Vlastimil Babka wrote:
> > +static bool
> > +__reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
> > + bool check_target)
> > +{
> > + struct page *page = pfn_to_online_page(pfn);
> > + struct page *end_page;
> > +
> > + if (!page)
> > + return false;
> > + if (zone != page_zone(page))
> > + return false;
> > + if (pageblock_skip_persistent(page))
> > + return false;
> > +
> > + /*
> > + * If skip is already cleared do no further checking once the
> > + * restart points have been set.
> > + */
> > + if (check_source && check_target && !get_pageblock_skip(page))
> > + return true;
> > +
> > + /*
> > + * If clearing skip for the target scanner, do not select a
> > + * non-movable pageblock as the starting point.
> > + */
> > + if (!check_source && check_target &&
> > + get_pageblock_migratetype(page) != MIGRATE_MOVABLE)
> > + return false;
> > +
> > + /*
> > + * Only clear the hint if a sample indicates there is either a
> > + * free page or an LRU page in the block. One or other condition
> > + * is necessary for the block to be a migration source/target.
> > + */
> > + page = pfn_to_page(pageblock_start_pfn(pfn));
> > + if (zone != page_zone(page))
> > + return false;
> > + end_page = page + pageblock_nr_pages;
>
> Watch out for start pfn being invalid, and end_page being invalid or after zone end?
>

Yeah, it is possible there is no alignment on pageblock_nr_pages.

> > +
> > + do {
> > + if (check_source && PageLRU(page)) {
> > + clear_pageblock_skip(page);
> > + return true;
> > + }
> > +
> > + if (check_target && PageBuddy(page)) {
> > + clear_pageblock_skip(page);
> > + return true;
> > + }
> > +
> > + page += (1 << PAGE_ALLOC_COSTLY_ORDER);
>
> Also probably check pfn_valid_within() and page_zone?
>

Again yes. Holes could have been punched.

I've an updated version but I'll shove it through tests just to be sure.

> > + } while (page < end_page);
> > +
> > + return false;
> > +}
> > +
> > /*
> > * 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
>
> ...
>
> > @@ -1193,7 +1273,7 @@ fast_isolate_freepages(struct compact_control *cc)
> > * If starting the scan, use a deeper search and use the highest
> > * PFN found if a suitable one is not found.
> > */
> > - if (cc->free_pfn == pageblock_start_pfn(zone_end_pfn(cc->zone) - 1)) {
> > + if (cc->free_pfn >= cc->zone->compact_init_free_pfn) {
> > limit = pageblock_nr_pages >> 1;
> > scan_start = true;
> > }
> > @@ -1338,7 +1418,6 @@ static void isolate_freepages(struct compact_control *cc)
> > unsigned long isolate_start_pfn; /* exact pfn we start at */
> > unsigned long block_end_pfn; /* end of current pageblock */
> > unsigned long low_pfn; /* lowest pfn scanner is able to scan */
> > - unsigned long nr_isolated;
> > struct list_head *freelist = &cc->freepages;
> > unsigned int stride;
> >
> > @@ -1374,6 +1453,8 @@ static void isolate_freepages(struct compact_control *cc)
> > block_end_pfn = block_start_pfn,
> > block_start_pfn -= pageblock_nr_pages,
> > isolate_start_pfn = block_start_pfn) {
> > + unsigned long nr_isolated;
>
> Unrelated cleanup? Nevermind.
>

I'll move the hunks to "mm, compaction: Sample pageblocks for free
pages" where they belong

--
Mel Gorman
SUSE Labs

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