Re: [PATCH] vmscan: memcg needs may_swap (Re: [patch] vmscan: rename sc.may_swap to may_unmap)

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 31, 2009 at 10:48:32AM +0900, KOSAKI Motohiro wrote:
> > > Sorry for too late response.
> > > I don't know memcg well.
> > > 
> > > The memcg managed to use may_swap well with global page reclaim until now.
> > > I think that was because may_swap can represent both meaning.
> > > Do we need each variables really ?
> > > 
> > > How about using union variable ?
> > 
> > or Just removing one of them  ?
> 
> I hope all may_unmap user convert to using may_swap.
> may_swap is more efficient and cleaner meaning.

How about making may_swap mean the following:

	@@ -642,6 +639,8 @@ static unsigned long shrink_page_list(st
	 		 * Try to allocate it some swap space here.
	 		 */
	 		if (PageAnon(page) && !PageSwapCache(page)) {
	+			if (!sc->map_swap)
	+				goto keep_locked;
	 			if (!(sc->gfp_mask & __GFP_IO))
	 				goto keep_locked;
	 			if (!add_to_swap(page))

try_to_free_pages() always sets it.

try_to_free_mem_cgroup_pages() sets it depending on whether it really
wants swapping, and only swapping, right?  But the above would still
reclaim already swapped anon pages and I don't know the memory
controller.

balance_pgdat() always sets it.

__zone_reclaim() sets it depending on zone_reclaim_mode.  The
RECLAIM_SWAP bit of this field and its documentation in
Documentation/sysctl/vm.txt suggests it also really only means swap.

shrink_all_memory() would be the sole user of may_unmap because it
really wants to eat cache first.  But this could be figured out on a
different occasion.

	Hannes