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SubjectRe: [PATCH 09/22] mm, compaction: Use free lists to quickly locate a migration source
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On 1/18/19 6:51 PM, Mel Gorman wrote:
...

> + for (order = cc->order - 1;
> + order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit;
> + order--) {
> + struct free_area *area = &cc->zone->free_area[order];
> + struct list_head *freelist;
> + unsigned long flags;
> + struct page *freepage;
> +
> + if (!area->nr_free)
> + continue;
> +
> + spin_lock_irqsave(&cc->zone->lock, flags);
> + freelist = &area->free_list[MIGRATE_MOVABLE];
> + list_for_each_entry(freepage, freelist, lru) {
> + unsigned long free_pfn;
> +
> + nr_scanned++;
> + free_pfn = page_to_pfn(freepage);
> + if (free_pfn < high_pfn) {
> + update_fast_start_pfn(cc, free_pfn);

Shouldn't this update go below checking pageblock skip bit? We might be
caching pageblocks that will be skipped, and also potentially going
backwards from the original cc->migrate_pfn, which could perhaps explain
the reported kcompactd loops?

> +
> + /*
> + * Avoid if skipped recently. Ideally it would
> + * move to the tail but even safe iteration of
> + * the list assumes an entry is deleted, not
> + * reordered.
> + */
> + if (get_pageblock_skip(freepage)) {
> + if (list_is_last(freelist, &freepage->lru))
> + break;
> +
> + continue;
> + }
> +
> + /* Reorder to so a future search skips recent pages */
> + move_freelist_tail(freelist, freepage);
> +
> + pfn = pageblock_start_pfn(free_pfn);
> + cc->fast_search_fail = 0;
> + set_pageblock_skip(freepage);
> + break;
> + }
> +
> + if (nr_scanned >= limit) {
> + cc->fast_search_fail++;
> + move_freelist_tail(freelist, freepage);
> + break;
> + }
> + }
> + spin_unlock_irqrestore(&cc->zone->lock, flags);
> + }
> +
> + cc->total_migrate_scanned += nr_scanned;
> +
> + /*
> + * If fast scanning failed then use a cached entry for a page block
> + * that had free pages as the basis for starting a linear scan.
> + */
> + if (pfn == cc->migrate_pfn)
> + reinit_migrate_pfn(cc);

This will set cc->migrate_pfn to the lowest pfn encountered, yet return
pfn initialized by original cc->migrate_pfn.
AFAICS isolate_migratepages() will use the returned pfn for the linear
scan and then overwrite cc->migrate_pfn with wherever it advanced from
there. So whatever we stored here into cc->migrate_pfn will never get
actually used, except when isolate_migratepages() returns with
ISOLATED_ABORT.
So maybe the infinite kcompactd loop is linked to ISOLATED_ABORT?

> +
> + return pfn;
> +}
> +
> /*
> * Isolate all pages that can be migrated from the first suitable block,
> * starting at the block pointed to by the migrate scanner pfn within
> @@ -1222,16 +1381,25 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
> const isolate_mode_t isolate_mode =
> (sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
> (cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);
> + bool fast_find_block;
>
> /*
> * Start at where we last stopped, or beginning of the zone as
> - * initialized by compact_zone()
> + * initialized by compact_zone(). The first failure will use
> + * the lowest PFN as the starting point for linear scanning.
> */
> - low_pfn = cc->migrate_pfn;
> + low_pfn = fast_find_migrateblock(cc);
> block_start_pfn = pageblock_start_pfn(low_pfn);
> if (block_start_pfn < zone->zone_start_pfn)
> block_start_pfn = zone->zone_start_pfn;
>
> + /*
> + * fast_find_migrateblock marks a pageblock skipped so to avoid
> + * the isolation_suitable check below, check whether the fast
> + * search was successful.
> + */
> + fast_find_block = low_pfn != cc->migrate_pfn && !cc->fast_search_fail;
> +
> /* Only scan within a pageblock boundary */
> block_end_pfn = pageblock_end_pfn(low_pfn);
>
> @@ -1240,6 +1408,7 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
> * Do not cross the free scanner.
> */
> for (; block_end_pfn <= cc->free_pfn;
> + fast_find_block = false,
> low_pfn = block_end_pfn,
> block_start_pfn = block_end_pfn,
> block_end_pfn += pageblock_nr_pages) {
> @@ -1259,7 +1428,7 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
> continue;
>
> /* If isolation recently failed, do not retry */
> - if (!isolation_suitable(cc, page))
> + if (!isolation_suitable(cc, page) && !fast_find_block)
> continue;
>
> /*
> @@ -1550,6 +1719,7 @@ static enum compact_result compact_zone(struct compact_control *cc)
> * want to compact the whole zone), but check that it is initialised
> * by ensuring the values are within zone boundaries.
> */
> + cc->fast_start_pfn = 0;
> if (cc->whole_zone) {
> cc->migrate_pfn = start_pfn;
> cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
> diff --git a/mm/internal.h b/mm/internal.h
> index 9b32f4cab0ae..983cb975545f 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -188,9 +188,11 @@ struct compact_control {
> unsigned int nr_migratepages; /* Number of pages to migrate */
> unsigned long free_pfn; /* isolate_freepages search base */
> unsigned long migrate_pfn; /* isolate_migratepages search base */
> + unsigned long fast_start_pfn; /* a pfn to start linear scan from */
> struct zone *zone;
> unsigned long total_migrate_scanned;
> unsigned long total_free_scanned;
> + unsigned int fast_search_fail; /* failures to use free list searches */
> const gfp_t gfp_mask; /* gfp mask of a direct compactor */
> int order; /* order a direct compactor needs */
> int migratetype; /* migratetype of direct compactor */
>

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