Re: [PATCH] mm, compaction: Skip all pinned pages during scan

[Andrew Morton]

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