Re: [PATCH 08/13] mm/migrate: __unmap_and_move() push good newpage to LRU

[Vlastimil Babka]

On 2/6/22 22:43, Hugh Dickins wrote:
> Compaction, NUMA page movement, THP collapse/split, and memory failure
> do isolate unevictable pages from their "LRU", losing the record of
> mlock_count in doing so (isolators are likely to use page->lru for their
> own private lists, so mlock_count has to be presumed lost).
> 
> That's unfortunate, and we should put in some work to correct that: one
> can imagine a function to build up the mlock_count again - but it would
> require i_mmap_rwsem for read, so be careful where it's called.  Or
> page_referenced_one() and try_to_unmap_one() might do that extra work.
> 
> But one place that can very easily be improved is page migration's
> __unmap_and_move(): a small adjustment to where the successful new page
> is put back on LRU, and its mlock_count (if any) is built back up by
> remove_migration_ptes().
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/migrate.c | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 7c4223ce2500..f4bcf1541b62 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1032,6 +1032,21 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>  	if (!page_mapped(page))
>  		rc = move_to_new_page(newpage, page, mode);
>  
> +	/*
> +	 * When successful, push newpage to LRU immediately: so that if it
> +	 * turns out to be an mlocked page, remove_migration_ptes() will
> +	 * automatically build up the correct newpage->mlock_count for it.
> +	 *
> +	 * We would like to do something similar for the old page, when
> +	 * unsuccessful, and other cases when a page has been temporarily
> +	 * isolated from the unevictable LRU: but this case is the easiest.
> +	 */
> +	if (rc == MIGRATEPAGE_SUCCESS) {
> +		lru_cache_add(newpage);
> +		if (page_was_mapped)
> +			lru_add_drain();
> +	}
> +
>  	if (page_was_mapped)
>  		remove_migration_ptes(page,
>  			rc == MIGRATEPAGE_SUCCESS ? newpage : page, false);
> @@ -1045,20 +1060,12 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>  	unlock_page(page);
>  out:
>  	/*
> -	 * If migration is successful, decrease refcount of the newpage
> +	 * If migration is successful, decrease refcount of the newpage,
>  	 * which will not free the page because new page owner increased
> -	 * refcounter. As well, if it is LRU page, add the page to LRU
> -	 * list in here. Use the old state of the isolated source page to
> -	 * determine if we migrated a LRU page. newpage was already unlocked
> -	 * and possibly modified by its owner - don't rely on the page
> -	 * state.
> +	 * refcounter.
>  	 */
> -	if (rc == MIGRATEPAGE_SUCCESS) {
> -		if (unlikely(!is_lru))
> -			put_page(newpage);
> -		else
> -			putback_lru_page(newpage);
> -	}
> +	if (rc == MIGRATEPAGE_SUCCESS)
> +		put_page(newpage);
>  
>  	return rc;
>  }