Re: [PATCH v2 3/5] mm/page_alloc: move pages to tail in move_to_free_list()

[Vlastimil Babka <vbabka@xxxxxxx>]

On 10/5/20 2:15 PM, David Hildenbrand wrote:
> Whenever we move pages between freelists via move_to_free_list()/
> move_freepages_block(), we don't actually touch the pages:
> 1. Page isolation doesn't actually touch the pages, it simply isolates
>     pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.
>     When undoing isolation, we move the pages back to the target list.
> 2. Page stealing (steal_suitable_fallback()) moves free pages directly
>     between lists without touching them.
> 3. reserve_highatomic_pageblock()/unreserve_highatomic_pageblock() moves
>     free pages directly between freelists without touching them.
>
> We already place pages to the tail of the freelists when undoing isolation
> via __putback_isolated_page(), let's do it in any case (e.g., if order <=
> pageblock_order) and document the behavior. To simplify, let's move the
> pages to the tail for all move_to_free_list()/move_freepages_block() users.
>
> In 2., the target list is empty, so there should be no change. In 3.,
> we might observe a change, however, highatomic is more concerned about
> allocations succeeding than cache hotness - if we ever realize this
> change degrades a workload, we can special-case this instance and add a
> proper comment.
>
> This change results in all pages getting onlined via online_pages() to
> be placed to the tail of the freelist.
>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Acked-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>