linux-kernel - Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan

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Date:   Thu, 25 May 2023 20:58:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Khalid Aziz <khalid.aziz@...cle.com>
Cc:     akpm@...ux-foundation.org, steven.sistare@...cle.com,
        david@...hat.com, ying.huang@...el.com,
        mgorman@...hsingularity.net, baolin.wang@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Khalid Aziz <khalid@...nel.org>
Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during
 scan

On Thu, May 25, 2023 at 01:15:07PM -0600, Khalid Aziz wrote:
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 5a9501e0ae01..b548e05f0349 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -764,6 +764,42 @@ 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 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,
> +					struct address_space *mapping)
> +{
> +	unsigned long extra_refs;
> +	struct folio *folio;
> +
> +	/*
> +	 * Skip this check for pages in ZONE_MOVABLE or MIGRATE_CMA
> +	 * pages that can not be long term pinned
> +	 */
> +	if (is_zone_movable_page(page) || is_migrate_cma_page(page))
> +		return false;
> +
> +	folio = page_folio(page);
> +
> +	/*
> +	 * caller holds a ref already from get_page_unless_zero()
> +	 * which is accounted for in folio_expected_refs()
> +	 */
> +	extra_refs = folio_expected_refs(mapping, folio);
> +
> +	/*
> +	 * This is an admittedly racy check but good enough to determine
> +	 * if a page is pinned and can not be migrated
> +	 */
> +	if ((folio_ref_count(folio) - extra_refs) > folio_mapcount(folio))
> +		return true;
> +	return false;
> +}
> +
>  /**
>   * isolate_migratepages_block() - isolate all migrate-able pages within
>   *				  a single pageblock
> @@ -992,12 +1028,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  			goto isolate_fail;

Just out of shot, we have ...

                if (unlikely(!get_page_unless_zero(page)))

This is the perfect opportunity to use folio_get_nontail_page() instead.
You get back the folio without having to cast the pointer yourself
or call page_folio().  Now you can use a folio throughout your new
function, saving a call to compound_head().

For a followup patch, everything in this loop below this point can use
the folio ... that's quite a lot of change.

>  		/*
> -		 * 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
> +		 * from long term pinning preventing it from migrating,
> +		 * so avoid taking lru_lock and isolating it unnecessarily.
>  		 */

Isn't "long term pinning" the wrong description of the problem?  Long term
pins suggest to me FOLL_LONGTERM.  I think this is simple short term
pins that we care about here.