On Fri 02-11-18 19:38:35, Roman Gushchin wrote:
> On Fri, Nov 02, 2018 at 06:48:23PM +0100, Michal Hocko wrote:
> > On Fri 02-11-18 17:25:58, Roman Gushchin wrote:
> > > On Fri, Nov 02, 2018 at 05:51:47PM +0100, Michal Hocko wrote:
> > > > On Fri 02-11-18 16:22:41, Roman Gushchin wrote:
> > [...]
> > > > > 2) We do forget to scan the last page in the LRU list. So if we ended
> > > > > up with
> > > > > 1-page long LRU, it can stay there basically forever.
> > > >
> > > > Why
> > > > /*
> > > > * If the cgroup's already been deleted, make sure to
> > > > * scrape out the remaining cache.
> > > > */
> > > > if (!scan && !mem_cgroup_online(memcg))
> > > > scan = min(size, SWAP_CLUSTER_MAX);
> > > >
> > > > in get_scan_count doesn't work for that case?
> > >
> > > No, it doesn't. Let's look at the whole picture:
> > >
> > > size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
> > > scan = size >> sc->priority;
> > > /*
> > > * If the cgroup's already been deleted, make sure to
> > > * scrape out the remaining cache.
> > > */
> > > if (!scan && !mem_cgroup_online(memcg))
> > > scan = min(size, SWAP_CLUSTER_MAX);
> > >
> > > If size == 1, scan == 0 => scan = min(1, 32) == 1.
> > > And after proportional adjustment we'll have 0.
> >
> > My friday brain hurst when looking at this but if it doesn't work as
> > advertized then it should be fixed. I do not see any of your patches to
> > touch this logic so how come it would work after them applied?
>
> This part works as expected. But the following
> scan = div64_u64(scan * fraction[file], denominator);
> reliable turns 1 page to scan to 0 pages to scan.
OK, 68600f623d69 ("mm: don't miss the last page because of round-off
error") sounds like a good and safe stable backport material.
--
Michal Hocko
SUSE Labs
