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

[Kirill A. Shutemov]

On Tue, Oct 16, 2018 at 09:34:11AM +0000, Kirill Tkhai wrote:
> On 15.10.2018 18:41, Kirill A. Shutemov wrote:
> > On Thu, Oct 11, 2018 at 01:52:22PM +0300, Kirill Tkhai wrote:
> >> 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 |   15 +++++++++++++++
> >>  1 file changed, 15 insertions(+)
> >>
> >> diff --git a/mm/ksm.c b/mm/ksm.c
> >> index 5b0894b45ee5..b83ca37e28f0 100644
> >> --- a/mm/ksm.c
> >> +++ b/mm/ksm.c
> >> @@ -1321,6 +1321,21 @@ 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.
> > 
> > You cannot really check if the page is free. There are some paths that
> > makes the refcount zero temporarely, but doesn't free the page.
> > See page_ref_freeze() for instance.
> 
> Thanks. Does this look better?
> 
>   Find neighbour of @page containing 1-order pair in buddy-allocator
>   and check whether its count is 0. If it is so, we consider it's as free
>   (this is more probable than it's freezed via page_ref_freeze()),
>   and we try to use @tree_page as ksm page and to free @page.

Looks fine to me.

-- 
 Kirill A. Shutemov