Re: [PATCH] mm, compaction: fix NR_ISOLATED_* stats for pfn based migration

[Michal Hocko]

On Thu 20-10-16 20:16:06, Andrew Morton wrote:
> On Wed, 19 Oct 2016 10:02:40 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> 
> > Since bda807d44454 ("mm: migrate: support non-lru movable page
> > migration") isolate_migratepages_block) can isolate !PageLRU pages which
> > would acct_isolated account as NR_ISOLATED_*. Accounting these non-lru
> > pages NR_ISOLATED_{ANON,FILE} doesn't make any sense and it can misguide
> > heuristics based on those counters such as pgdat_reclaimable_pages resp.
> > too_many_isolated which would lead to unexpected stalls during the
> > direct reclaim without any good reason. Note that
> > __alloc_contig_migrate_range can isolate a lot of pages at once.
> > 
> > On mobile devices such as 512M ram android Phone, it may use a big zram
> > swap. In some cases zram(zsmalloc) uses too many non-lru but migratedable
> > pages, such as:
> > 
> >       MemTotal: 468148 kB
> >       Normal free:5620kB
> >       Free swap:4736kB
> >       Total swap:409596kB
> >       ZRAM: 164616kB(zsmalloc non-lru pages)
> >       active_anon:60700kB
> >       inactive_anon:60744kB
> >       active_file:34420kB
> >       inactive_file:37532kB
> > 
> > Fix this by only accounting lru pages to NR_ISOLATED_* in
> > isolate_migratepages_block right after they were isolated and we still
> > know they were on LRU. Drop acct_isolated because it is called after the
> > fact and we've lost that information. Batching per-cpu counter doesn't
> > make much improvement anyway. Also make sure that we uncharge only LRU
> > pages when putting them back on the LRU in putback_movable_pages resp.
> > when unmap_and_move migrates the page.
> 
> It isn't worth backporting into 4.8.x?

To be honest, I don't know. AFAIK nobody has ever seen any real problem
yet. So this is a just-in-case fix.

-- 
Michal Hocko
SUSE Labs