Subject: Re: [PATCH 2/4] mm/slub: Use mem_node to allocate a new slab

From: Srikar Dronamraju <[email-masked]>
Date: Tue, 17 Mar 2020 19:15:23 +0530

* Vlastimil Babka <[email-masked]> [2020-03-17 14:34:25]:

> On 3/17/20 2:17 PM, Srikar Dronamraju wrote:
> > Currently while allocating a slab for a offline node, we use its
> > associated node_numa_mem to search for a partial slab. If we don't find
> > a partial slab, we try allocating a slab from the offline node using
> > __alloc_pages_node. However this is bound to fail.
> > 
> > NIP [c00000000039a300] __alloc_pages_nodemask+0x130/0x3b0
> > LR [c00000000039a3c4] __alloc_pages_nodemask+0x1f4/0x3b0
> > Call Trace:
> > [c0000008b36837f0] [c00000000039a3b4] __alloc_pages_nodemask+0x1e4/0x3b0 (unreliable)
> > [c0000008b3683870] [c0000000003d1ff8] new_slab+0x128/0xcf0
> > [c0000008b3683950] [c0000000003d6060] ___slab_alloc+0x410/0x820
> > [c0000008b3683a40] [c0000000003d64a4] __slab_alloc+0x34/0x60
> > [c0000008b3683a70] [c0000000003d78b0] __kmalloc_node+0x110/0x490
> > [c0000008b3683af0] [c000000000343a08] kvmalloc_node+0x58/0x110
> > [c0000008b3683b30] [c0000000003ffd44] mem_cgroup_css_online+0x104/0x270
> > [c0000008b3683b90] [c000000000234e08] online_css+0x48/0xd0
> > [c0000008b3683bc0] [c00000000023dedc] cgroup_apply_control_enable+0x2ec/0x4d0
> > [c0000008b3683ca0] [c0000000002416f8] cgroup_mkdir+0x228/0x5f0
> > [c0000008b3683d10] [c000000000520360] kernfs_iop_mkdir+0x90/0xf0
> > [c0000008b3683d50] [c00000000043e400] vfs_mkdir+0x110/0x230
> > [c0000008b3683da0] [c000000000441ee0] do_mkdirat+0xb0/0x1a0
> > [c0000008b3683e20] [c00000000000b278] system_call+0x5c/0x68
> > 
> > Mitigate this by allocating the new slab from the node_numa_mem.
> 
> Are you sure this is really needed and the other 3 patches are not enough for
> the current SLUB code to work as needed? It seems you are changing the semantics
> here...
> 

The other 3 patches are not enough because we don't carry the searchnode
when the actual alloc_pages_node gets called. 

With only the 3 patches, we see the above Panic, its signature is slightly
different from what Sachin first reported and which I have carried in 1st
patch.

> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -1970,14 +1970,8 @@ static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
> >  		struct kmem_cache_cpu *c)
> >  {
> >  	void *object;
> > -	int searchnode = node;
> >  
> > -	if (node == NUMA_NO_NODE)
> > -		searchnode = numa_mem_id();
> > -	else if (!node_present_pages(node))
> > -		searchnode = node_to_mem_node(node);
> > -
> > -	object = get_partial_node(s, get_node(s, searchnode), c, flags);
> > +	object = get_partial_node(s, get_node(s, node), c, flags);
> >  	if (object || node != NUMA_NO_NODE)>  		return object;
> >
> >       return get_any_partial(s, flags, c);
> 
> I.e. here in this if(), now node will never equal NUMA_NO_NODE (thanks to the
> hunk below), thus the get_any_partial() call becomes dead code?
> 
> > @@ -2470,6 +2464,11 @@ static inline void *new_slab_objects(struct kmem_cache *s, gfp_t flags,
> >  
> >  	WARN_ON_ONCE(s->ctor && (flags & __GFP_ZERO));
> >  
> > +	if (node == NUMA_NO_NODE)
> > +		node = numa_mem_id();
> > +	else if (!node_present_pages(node))
> > +		node = node_to_mem_node(node);
> > +
> >  	freelist = get_partial(s, flags, node, c);
> >  
> >  	if (freelist)
> > @@ -2569,12 +2568,10 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> >  redo:
> >  
> >  	if (unlikely(!node_match(page, node))) {
> > -		int searchnode = node;
> > -
> >  		if (node != NUMA_NO_NODE && !node_present_pages(node))
> > -			searchnode = node_to_mem_node(node);
> > +			node = node_to_mem_node(node);
> >  
> > -		if (unlikely(!node_match(page, searchnode))) {
> > +		if (unlikely(!node_match(page, node))) {
> >  			stat(s, ALLOC_NODE_MISMATCH);
> >  			deactivate_slab(s, page, c->freelist, c);
> >  			goto new_slab;
> > 
> 

-- 
Thanks and Regards
Srikar Dronamraju