On Thu, Sep 03, 2020 at 09:10:59PM -0700, Andrew Morton wrote:
> On Thu, 3 Sep 2020 16:00:55 -0700 Roman Gushchin <g...@fb.com> wrote:
>
> > In the memcg case count_shadow_nodes() sums the number of pages in lru
> > lists and the amount of slab memory (reclaimable and non-reclaimable)
> > as a baseline for the allowed number of shadow entries.
> >
> > It seems to be a good analogy for the !memcg case, where
> > node_present_pages() is used. However, it's not quite true, as there
> > two problems:
> >
> > 1) Due to slab reparenting introduced by commit fb2f2b0adb98 ("mm:
> > memcg/slab: reparent memcg kmem_caches on cgroup removal") local
> > per-lruvec slab counters might be inaccurate on non-leaf levels.
> > It's the only place where local slab counters are used.
> >
> > 2) Shadow nodes by themselves are backed by slabs. So there is a loop
> > dependency: the more shadow entries are there, the less pressure the
> > kernel applies to reclaim them.
> >
> > Fortunately, there is a simple way to solve both problems: slab
> > counters shouldn't be taken into the account by count_shadow_nodes().
> >
> > ...
> >
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -495,10 +495,6 @@ static unsigned long count_shadow_nodes(struct
> > shrinker *shrinker,
> > for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
> > pages += lruvec_page_state_local(lruvec,
> > NR_LRU_BASE + i);
> > - pages += lruvec_page_state_local(
> > - lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT;
> > - pages += lruvec_page_state_local(
> > - lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT;
> > } else
> > #endif
> > pages = node_present_pages(sc->nid);
>
> Did this have any observable runtime effects?
Most likely not.
I maybe saw the second effect once, but it was backed up by a bug in the inode
reclaim path in the exact kernel version I used (not an upstream one).
The first problem is pure theoretical, I'm just not comfortable with using these
counters, which are known to be inaccurate after reparenting.
That's why I didn't add stable@.
Thanks!
