Re: [PATCH v15 3/7] mm: Add function __putback_isolated_page

David Hildenbrand
2019-12-16T11:36:37Z

[...]
> +/**
> + * __putback_isolated_page - Return a now-isolated page back where we got it
> + * @page: Page that was isolated
> + * @order: Order of the isolated page
> + *
> + * This function is meant to return a page pulled from the free lists via
> + * __isolate_free_page back to the free lists they were pulled from.
> + */
> +void __putback_isolated_page(struct page *page, unsigned int order)
> +{
> + struct zone *zone = page_zone(page);
> + unsigned long pfn;
> + unsigned int mt;
> +
> + /* zone lock should be held when this function is called */
> + lockdep_assert_held(&zone->lock);
> +
> + pfn = page_to_pfn(page);
> + mt = get_pfnblock_migratetype(page, pfn);

IMHO get_pageblock_migratetype() would be nicer - I guess the compiler
will optimize out the double page_to_pfn().

> +
> + /* Return isolated page to tail of freelist. */
> + __free_one_page(page, pfn, zone, order, mt);
> +}
> +
> /*
> * Update NUMA hit/miss statistics
> *
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 04ee1663cdbe..d93d2be0070f 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -134,13 +134,11 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
> __mod_zone_freepage_state(zone, nr_pages, migratetype);
> }
> set_pageblock_migratetype(page, migratetype);
> + if (isolated_page)
> + __putback_isolated_page(page, order);
> zone->nr_isolate_pageblock--;
> out:
> spin_unlock_irqrestore(&zone->lock, flags);
> - if (isolated_page) {
> - post_alloc_hook(page, order, __GFP_MOVABLE);
> - __free_pages(page, order);
> - }

So If I get it right:

post_alloc_hook() does quite some stuff like
- arch_alloc_page(page, order);
- kernel_map_pages(page, 1 << order, 1)
- kasan_alloc_pages()
- kernel_poison_pages(1)
- set_page_owner()

Which free_pages_prepare() will undo, like
- reset_page_owner()
- kernel_poison_pages(0)
- arch_free_page()
- kernel_map_pages()
- kasan_free_nondeferred_pages()

Both would be skipped now - which sounds like the right thing to do IMHO
(and smells like quite a performance improvement). I haven't verified if
actually everything we skip in free_pages_prepare() is safe (I think it
is, it seems to be mostly relevant for actually used/allocated pages).

--
Thanks,

David / dhildenb