On Tue 28-01-20 12:39:55, Michal Hocko wrote: > On Tue 28-01-20 02:48:57, Matthew Wilcox wrote: > > On Tue, Jan 28, 2020 at 10:13:52AM +0100, Michal Hocko wrote: > > > On Tue 28-01-20 00:30:44, Matthew Wilcox wrote: > > > > On Tue, Jan 28, 2020 at 09:17:12AM +0100, Michal Hocko wrote: > > > > > On Mon 27-01-20 11:06:53, Matthew Wilcox wrote: > > > > > > On Mon, Jan 27, 2020 at 04:00:24PM +0100, Michal Hocko wrote: > > > > > > > On Sun 26-01-20 15:39:35, Matthew Wilcox wrote: > > > > > > > > On Sun, Jan 26, 2020 at 11:53:55AM -0800, Cong Wang wrote: > > > > > > > > > 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 > > > > > > > > > > I am not deeply familiar with the readahead code. But is there really a > > > > > high oerder allocation (order > 1) that would trigger compaction in the > > > > > phase when pages are locked? > > > > > > > > Thanks to sl*b, yes: > > > > > > > > radix_tree_node 80890 102536 584 28 4 : tunables 0 0 0 : slabdata 3662 3662 0 > > > > > > > > so it's allocating 4 pages for an allocation of a 576 byte node. > > > > > > I am not really sure that we do sync migration for costly orders. > > > > Doesn't the stack trace above indicate that we're doing migration as > > the result of an allocation in add_to_page_cache_lru()? > > Which stack trace do you refer to? Because the one above doesn't show > much more beyond mem_cgroup_iter and likewise others in this email > thread. I do not really remember any stack with lock_page on the trace. > > > > > > > Btw. the compaction rejects to consider file backed pages when __GFP_FS > > > > > is not present AFAIR. > > > > > > > > Ah, that would save us. > > > > > > So the NOFS comes from the mapping GFP mask, right? That is something I > > > was hoping to get rid of eventually :/ Anyway it would be better to have > > > an explicit NOFS with a comment explaining why we need that. If for > > > nothing else then for documentation. > > > > I'd also like to see the mapping GFP mask go away, but rather than seeing > > an explicit GFP_NOFS here, I'd rather see the memalloc_nofs API used. > > Completely agreed agree here. The proper place for the scope would be > the place where pages are locked with an explanation that there are > other allocations down the line which might invoke sync migration and > that would be dangerous. Having that explicitly documented is clearly an > improvement. Can we pursue on this please? An explicit NOFS scope annotation with a reference to compaction potentially locking up on pages in the readahead would be a great start. -- Michal Hocko SUSE Labs
