On Thu, 11 May 2023 10:55:16 -0600 Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
> Pinned pages can not be migrated. Currently as
> isolate_migratepages_block() scans pages for compaction, it skips
> any pinned anonymous pages. All pinned pages should be skipped and
> not just the anonymous pinned pages. This patch adds a check for
> pinned page by comparing its refcount with mapcount and accounts for
> possible extra refcounts. This was seen as a real issue on a
> customer workload where a large number of pages were pinned by vfio
> on the host and any attempts to allocate hugepages resulted in
> significant amount of cpu time spent in either direct compaction or
> in kcompatd scanning vfio pinned pages over and over again that can
> not be migrated.
>
> Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> ---
> mm/compaction.c | 33 +++++++++++++++++++++++++++++----
> 1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5a9501e0ae01..d1371fd75391 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -764,6 +764,32 @@ static bool too_many_isolated(pg_data_t *pgdat)
> return too_many;
> }
>
> +/*
> + * Check if this base page should be skipped from isolation because
> + * it is pinned. This function is called for regular pages only, and not
> + * for THP or hugetlbfs pages. This code is inspired by similar code
> + * in migrate_vma_check_page(), can_split_folio() and
> + * folio_migrate_mapping()
> + */
> +static inline bool is_pinned_page(struct page *page)
> +{
> + unsigned long extra_refs;
> +
> + /* anonymous page can have extra ref from page cache */
"from swapcache"?
> + if (page_mapping(page))
> + extra_refs = 1 + page_has_private(page);
> + else
> + extra_refs = PageSwapCache(page) ? 1 : 0;
> +
> + /*
> + * This is an admittedly racy check but good enough to determine
> + * if a page should be isolated
"cannot be migrated"?
> + */
> + if ((page_count(page) - extra_refs) > page_mapcount(page))
> + return true;
> + return false;
> +}
> +
> /**
> * isolate_migratepages_block() - isolate all migrate-able pages within
> * a single pageblock
> @@ -992,12 +1018,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> goto isolate_fail;
>
> /*
> - * Migration will fail if an anonymous page is pinned in memory,
> - * so avoid taking lru_lock and isolating it unnecessarily in an
> - * admittedly racy check.
> + * Migration will fail if a page is pinned in memory,
> + * so avoid taking lru_lock and isolating it unnecessarily
> */
> mapping = page_mapping(page);
> - if (!mapping && (page_count(page) - 1) > total_mapcount(page))
> + if (is_pinned_page(page))
> goto isolate_fail_put;
>
> /*
> --
> 2.37.2
