Re: [PATCH v3] mm, compaction: Skip all non-migratable pages during scan

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 5/18/2023 12:15 AM, Khalid Aziz wrote:
> Pages pinned in memory through extra refcounts can not be migrated.
> Currently as isolate_migratepages_block() scans pages for
> compaction, it skips any pinned anonymous pages. All non-migratable
> pages should be skipped and not just the anonymous pinned pages.
> This patch adds a check for extra refcounts on a page to determine
> if the page can be migrated.  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 kcompactd scanning vfio pinned pages over and over again that can
> not be migrated.
> 
> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
> Suggested-by: Steve Sistare <steven.sistare@oracle.com>
> ---
> v3:
> 	- Account for extra ref added by get_page_unless_zero() earlier
> 	  in isolate_migratepages_block() (Suggested by Huang, Ying)
> 	- Clean up computation of extra refs to be consistent
> 	  (Suggested by Huang, Ying)
> 
> v2:
> 	- Update comments in the code (Suggested by Andrew)
> 	- Use PagePrivate() instead of page_has_private() (Suggested
> 	  by Matthew)
> 	- Pass mapping to page_has_extrarefs() (Suggested by Matthew)
> 	- Use page_ref_count() (Suggested by Matthew)
> 	- Rename is_pinned_page() to reflect its function more
> 	  accurately (Suggested by Matthew)
> 
>   mm/compaction.c | 36 ++++++++++++++++++++++++++++++++----
>   1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5a9501e0ae01..f04c00981172 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -764,6 +764,34 @@ static bool too_many_isolated(pg_data_t *pgdat)
>   	return too_many;
>   }
>   
> +/*
> + * Check if this base page should be skipped from isolation because
> + * it has extra refcounts that will prevent it from being migrated.
> + * 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 page_has_extra_refs(struct page *page)
> +{
> +	/* caller holds a ref already from get_page_unless_zero() */
> +	unsigned long extra_refs = 1;
> +
> +	/* anonymous page can have extra ref from swap cache */
> +	if (PageAnon(page))
> +		extra_refs += PageSwapCache(page) ? 1 : 0;
> +	else
> +		extra_refs += 1 + PagePrivate(page);
> +
> +	/*
> +	 * This is an admittedly racy check but good enough to determine
> +	 * if a page is pinned and can not be migrated
> +	 */
> +	if ((page_ref_count(page) - extra_refs) > page_mapcount(page))

Could you explain why changing total_mapcount() to page_mapcount()? The 
original commit 829ae0f81ce0 ("mm: migrate: fix THP's mapcount on 
isolation") did fix a THP's mapcount issue with converting to 
total_mapcount().

> +		return true;
> +	return false;
> +}
> +
>   /**
>    * isolate_migratepages_block() - isolate all migrate-able pages within
>    *				  a single pageblock
> @@ -992,12 +1020,12 @@ 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 has extra refcounts
> +		 * preventing it from migrating, so avoid taking
> +		 * lru_lock and isolating it unnecessarily
>   		 */
>   		mapping = page_mapping(page);
> -		if (!mapping && (page_count(page) - 1) > total_mapcount(page))
> +		if (page_has_extra_refs(page))
>   			goto isolate_fail_put;
>   
>   		/*