On 15.10.2018 13:38, Claudio Imbrenda wrote: > 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?
I have no synthetic tests on compaction, so this patch is RFC. Maybe you suggest something? In my test machine I added debug patch on top of this, which adds a counter of such tree_page preferred pages, and the counter increments as well. Order-1 page is just a brick of a bigger order pages, so this patch cares about them. > > On Mon, 15 Oct 2018 12:33:36 +0300 > Kirill Tkhai <[email protected]> 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 <[email protected]> >> --- >> 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, >> >
