Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

[Michal Hocko]

On Sun 26-01-20 15:39:35, Matthew Wilcox wrote:
> On Sun, Jan 26, 2020 at 11:53:55AM -0800, Cong Wang wrote:
> > On Tue, Jan 21, 2020 at 1:00 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > On Mon 20-01-20 14:48:05, Cong Wang wrote:
> > > > It got stuck somewhere along the call path of mem_cgroup_try_charge(),
> > > > and the trace events of mm_vmscan_lru_shrink_inactive() indicates this
> > > > too:
> > >
> > > So it seems that you are condending on the page lock. It is really
> > > unexpected that the reclaim would take that long though. Please try to
> > > enable more vmscan tracepoints to see where the time is spent.
> > 
> > I suspect the process gets stuck in the retry loop in try_charge(), as
> > the _shortest_ stacktrace of the perf samples indicated:
> > 
> > cycles:ppp:
> >         ffffffffa72963db mem_cgroup_iter
> >         ffffffffa72980ca mem_cgroup_oom_unlock
> >         ffffffffa7298c15 try_charge
> >         ffffffffa729a886 mem_cgroup_try_charge
> >         ffffffffa720ec03 __add_to_page_cache_locked
> >         ffffffffa720ee3a add_to_page_cache_lru
> >         ffffffffa7312ddb iomap_readpages_actor
> >         ffffffffa73133f7 iomap_apply
> >         ffffffffa73135da iomap_readpages
> >         ffffffffa722062e read_pages
> >         ffffffffa7220b3f __do_page_cache_readahead
> >         ffffffffa7210554 filemap_fault
> >         ffffffffc039e41f __xfs_filemap_fault
> >         ffffffffa724f5e7 __do_fault
> >         ffffffffa724c5f2 __handle_mm_fault
> >         ffffffffa724cbc6 handle_mm_fault
> >         ffffffffa70a313e __do_page_fault
> >         ffffffffa7a00dfe page_fault
> > 
> > But I don't see how it could be, the only possible case is when
> > mem_cgroup_oom() returns OOM_SUCCESS. However I can't
> > find any clue in dmesg pointing to OOM. These processes in the
> > same memcg are either running or sleeping (that is not exiting or
> > coredump'ing), I don't see how and why they could be selected as
> > a victim of OOM killer. I don't see any signal pending either from
> > their /proc/X/status.
> 
> I think this is a situation where we might end up with a genuine deadlock
> if we're not trylocking the pages.  readahead allocates a batch of
> locked pages and adds them to the pagecache.  If it has allocated,
> say, 5 pages, successfully inserted the first three into i_pages, then
> needs to allocate memory to insert the fourth one into i_pages, and
> the process then attempts to migrate the pages which are still locked,
> they will never come unlocked because they haven't yet been submitted
> to the filesystem for reading.

Just to make sure I understand. Do you mean this?
lock_page(A)
alloc_pages
  try_to_compact_pages
    compact_zone_order
      compact_zone(MIGRATE_SYNC_LIGHT)
        migrate_pages
	  unmap_and_move
	    __unmap_and_move
	      lock_page(A)
-- 
Michal Hocko
SUSE Labs