Re: [PATCH v2] mm/vmscan: shrink slab in node reclaim

[Michal Hocko <mhocko@kernel.org>]

On Tue 06-08-19 09:35:25, Michal Hocko wrote:
> On Tue 06-08-19 03:19:00, Yafang Shao wrote:
> > In the node reclaim, may_shrinkslab is 0 by default,
> > hence shrink_slab will never be performed in it.
> > While shrik_slab should be performed if the relcaimable slab is over
> > min slab limit.
> > 
> > Add scan_control::no_pagecache so shrink_node can decide to reclaim page
> > cache, slab, or both as dictated by min_unmapped_pages and min_slab_pages.
> > shrink_node will do at least one of the two because otherwise node_reclaim
> > returns early.
> > 
> > __node_reclaim can detect when enough slab has been reclaimed because
> > sc.reclaim_state.reclaimed_slab will tell us how many pages are
> > reclaimed in shrink slab.
> > 
> > This issue is very easy to produce, first you continuously cat a random
> > non-exist file to produce more and more dentry, then you read big file
> > to produce page cache. And finally you will find that the denty will
> > never be shrunk in node reclaim (they can only be shrunk in kswapd until
> > the watermark is reached).
> > 
> > Regarding vm.zone_reclaim_mode, we always set it to zero to disable node
> > reclaim. Someone may prefer to enable it if their different workloads work
> > on different nodes.
> 
> Considering that this is a long term behavior of a rarely used node
> reclaim I would rather not touch it unless some _real_ workload suffers
> from this behavior. Or is there any reason to fix this even though there
> is no evidence of real workloads suffering from the current behavior?

I have only now noticed that you have added
Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")

could you be more specific how that commit introduced a bug in the node
reclaim? It has introduced may_shrink_slab but the direct reclaim seems
to set it to 1.
-- 
Michal Hocko
SUSE Labs