Re: [PATCH 0/3] memdelay: memory health metric for systems and workloads

[Michal Hocko <mhocko@kernel.org>]

On Fri 28-07-17 15:43:37, Johannes Weiner wrote:
> Hi Andrew,
> 
> On Thu, Jul 27, 2017 at 01:43:25PM -0700, Andrew Morton wrote:
> > On Thu, 27 Jul 2017 11:30:07 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> > 
> > > This patch series implements a fine-grained metric for memory
> > > health.
> > 
> > I assume some Documentation/ is forthcoming.
> 
> Yep, I'll describe the interface and how to use this more extensively.
> 
> > Consuming another page flag hurts.  What's our current status there?
> 
> I would say we can make it 64-bit only, but I also need this refault
> distinction flag in the LRU balancing patches [1] to apply pressure on
> anon pages only when the page cache is actually thrashing, not when
> it's just transitioning to another workingset. So let's see...

I didn't get to look at the patchset yet but just for this part. I guess
you can go without a new page flag. PG_slab could be reused with some
care AFAICS.  Slab allocators do not seem to use other page flags so we
could make

bool PageSlab() 
{
	unsigned long flags = page->flags & ((1UL << NR_PAGEFLAGS) - 1);
	return (flags & (1UL << PG_slab)) == (1UL << PG_slab);
}

and then reuse the same bit for working set pages. Page cache will
almost always have LRU bit set and workingset_eviction assumes PG_locked
so we will have another bit set when needed. I know this is fuggly and
subtle but basically everything about struct page is inevitably like
that...
-- 
Michal Hocko
SUSE Labs