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

[Vlastimil Babka]

On 3/17/20 2:45 PM, Srikar Dronamraju wrote:
> * Vlastimil Babka <vbabka at suse.cz> [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.

Ah, I see. So that's the missing pgdat after your series [1] right?

That sounds like an argument for Michal's suggestions that pgdats exist and have
correctly populated zonelists for all possible nodes.
node_to_mem_node() could be just a shortcut for the first zone's node in the
zonelist, so that fallback follows the topology.

[1]
https://lore.kernel.org/linuxppc-dev/20200311110237.5731-1-srikar@linux.vnet.ibm.com/t/#m76e5b4c4084380b1d4b193d5aa0359b987f2290e

>> > --- 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;
>> > 
>> 
>