Re: [PATCH RFC v2] ksm: Assist buddy allocator to assemble 1-order pages

[Claudio Imbrenda]

I don't have objections to this patch, but I wonder how much impact it
would have. Have you performed any tests? does it really have such a big
impact on the availability of order-1 page blocks? 


Claudio

On Mon, 15 Oct 2018 12:33:36 +0300
Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:

> v2: Style improvements
> 
> try_to_merge_two_pages() merges two pages, one of them
> is a page of currently scanned mm, the second is a page
> with identical hash from unstable tree. Currently, we
> merge the page from unstable tree into the first one,
> and then free it.
> 
> The idea of this patch is to prefer freeing that page
> of them, which has a free neighbour (i.e., neighbour
> with zero page_count()). This allows buddy allocator
> to assemble at least 1-order set from the freed page
> and its neighbour; this is a kind of cheep passive
> compaction.
> 
> AFAIK, 1-order pages set consists of pages with PFNs
> [2n, 2n+1] (odd, even), so the neighbour's pfn is
> calculated via XOR with 1. We check the result pfn
> is valid and its page_count(), and prefer merging
> into @tree_page if neighbour's usage count is zero.
> 
> There a is small difference with current behavior
> in case of error path. In case of the second
> try_to_merge_with_ksm_page() is failed, we return
> from try_to_merge_two_pages() with @tree_page
> removed from unstable tree. It does not seem to matter,
> but if we do not want a change at all, it's not
> a problem to move remove_rmap_item_from_tree() from
> try_to_merge_with_ksm_page() to its callers.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> ---
>  mm/ksm.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 5b0894b45ee5..005508c86d0a 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -1321,6 +1321,22 @@ static struct page
> *try_to_merge_two_pages(struct rmap_item *rmap_item, {
>  	int err;
> 
> +	if (IS_ENABLED(CONFIG_COMPACTION)) {
> +		unsigned long pfn;
> +
> +		/*
> +		 * Find neighbour of @page containing 1-order pair
> +		 * in buddy-allocator and check whether it is free.
> +		 * If it is so, try to use @tree_page as ksm page
> +		 * and to free @page.
> +		 */
> +		pfn = page_to_pfn(page) ^ 1;
> +		if (pfn_valid(pfn) && page_count(pfn_to_page(pfn))
> == 0) {
> +			swap(rmap_item, tree_rmap_item);
> +			swap(page, tree_page);
> +		}
> +	}
> +
>  	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
>  	if (!err) {
>  		err = try_to_merge_with_ksm_page(tree_rmap_item,
>