Re: [RFC PATCH v4 7/7] mm/demotion: Demote pages according to allocation fallback order

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 6/6/22 1:12 PM, Ying Huang wrote:
> On Mon, 2022-06-06 at 11:51 +0530, Aneesh Kumar K.V wrote:
>> Ying Huang <ying.huang@intel.com> writes:
>>
>> .....
>>
>>>>>>
>>>>>>> https://lore.kernel.org/lkml/69f2d063a15f8c4afb4688af7b7890f32af55391.camel@intel.com/
>>>>>>>
>>>>>>> That is, something like below,
>>>>>>>
>>>>>>> static struct page *alloc_demote_page(struct page *page, unsigned long node)
>>>>>>> {
>>>>>>> 	struct page *page;
>>>>>>> 	nodemask_t allowed_mask;
>>>>>>> 	struct migration_target_control mtc = {
>>>>>>> 		/*
>>>>>>> 		 * Allocate from 'node', or fail quickly and quietly.
>>>>>>> 		 * When this happens, 'page' will likely just be discarded
>>>>>>> 		 * instead of migrated.
>>>>>>> 		 */
>>>>>>> 		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>>>>>>> 			    __GFP_THISNODE  | __GFP_NOWARN |
>>>>>>> 			    __GFP_NOMEMALLOC | GFP_NOWAIT,
>>>>>>> 		.nid = node
>>>>>>> 	};
>>>>>>>
>>>>>>> 	page = alloc_migration_target(page, (unsigned long)&mtc);
>>>>>>> 	if (page)
>>>>>>> 		return page;
>>>>>>>
>>>>>>> 	mtc.gfp_mask &= ~__GFP_THISNODE;
>>>>>>> 	mtc.nmask = &allowed_mask;
>>>>>>>
>>>>>>> 	return alloc_migration_target(page, (unsigned long)&mtc);
>>>>>>> }
>>>>>>
>>>>>> I skipped doing this in v5 because I was not sure this is really what we
>>>>>> want.
>>>>>
>>>>> I think so.  And this is the original behavior.  We should keep the
>>>>> original behavior as much as possible, then make changes if necessary.
>>>>>
>>>>
>>>> That is the reason I split the new page allocation as a separate patch.
>>>> Previous discussion on this topic didn't conclude on whether we really
>>>> need to do the above or not
>>>> https://lore.kernel.org/lkml/CAAPL-u9endrWf_aOnPENDPdvT-2-YhCAeJ7ONGckGnXErTLOfQ@mail.gmail.com/
>>>
>>> Please check the later email in the thread you referenced.  Both Wei and
>>> me agree that the use case needs to be supported.  We just didn't reach
>>> concensus about how to implement it.  If you think Wei's solution is
>>> better (referenced as below), you can try to do that too.  Although I
>>> think my proposed implementation is much simpler.
>>
>> How about the below details
>>
>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
>> index 79bd8d26feb2..cd6e71f702ad 100644
>> --- a/include/linux/memory-tiers.h
>> +++ b/include/linux/memory-tiers.h
>> @@ -21,6 +21,7 @@ void node_remove_from_memory_tier(int node);
>>   int node_get_memory_tier_id(int node);
>>   int node_set_memory_tier(int node, int tier);
>>   int node_reset_memory_tier(int node, int tier);
>> +void node_get_allowed_targets(int node, nodemask_t *targets);
>>   #else
>>   #define numa_demotion_enabled	false
>>   static inline int next_demotion_node(int node)
>> @@ -28,6 +29,10 @@ static inline int next_demotion_node(int node)
>>   	return NUMA_NO_NODE;
>>   }
>>   
>>
>>
>>
>> +static inline void node_get_allowed_targets(int node, nodemask_t *targets)
>> +{
>> +	*targets = NODE_MASK_NONE;
>> +}
>>   #endif	/* CONFIG_TIERED_MEMORY */
>>   
>>
>>
>>
>>   #endif
>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>> index b4e72b672d4d..592d939ec28d 100644
>> --- a/mm/memory-tiers.c
>> +++ b/mm/memory-tiers.c
>> @@ -18,6 +18,7 @@ struct memory_tier {
>>   
>>
>>
>>
>>   struct demotion_nodes {
>>   	nodemask_t preferred;
>> +	nodemask_t allowed;
>>   };
>>   
>>
>>
>>
>>   #define to_memory_tier(device) container_of(device, struct memory_tier, dev)
>> @@ -378,6 +379,25 @@ int node_set_memory_tier(int node, int tier)
>>   }
>>   EXPORT_SYMBOL_GPL(node_set_memory_tier);
>>   
>>
>>
>>
>> +void node_get_allowed_targets(int node, nodemask_t *targets)
>> +{
>> +	/*
>> +	 * node_demotion[] is updated without excluding this
>> +	 * function from running.
>> +	 *
>> +	 * If any node is moving to lower tiers then modifications
>> +	 * in node_demotion[] are still valid for this node, if any
>> +	 * node is moving to higher tier then moving node may be
>> +	 * used once for demotion which should be ok so rcu should
>> +	 * be enough here.
>> +	 */
>> +	rcu_read_lock();
>> +
>> +	*targets = node_demotion[node].allowed;
>> +
>> +	rcu_read_unlock();
>> +}
>> +
>>   /**
>>    * next_demotion_node() - Get the next node in the demotion path
>>    * @node: The starting node to lookup the next node
>> @@ -437,8 +457,10 @@ static void __disable_all_migrate_targets(void)
>>   {
>>   	int node;
>>   
>>
>>
>>
>> -	for_each_node_mask(node, node_states[N_MEMORY])
>> +	for_each_node_mask(node, node_states[N_MEMORY]) {
>>   		node_demotion[node].preferred = NODE_MASK_NONE;
>> +		node_demotion[node].allowed = NODE_MASK_NONE;
>> +	}
>>   }
>>   
>>
>>
>>
>>   static void disable_all_migrate_targets(void)
>> @@ -465,7 +487,7 @@ static void establish_migration_targets(void)
>>   	struct demotion_nodes *nd;
>>   	int target = NUMA_NO_NODE, node;
>>   	int distance, best_distance;
>> -	nodemask_t used;
>> +	nodemask_t used, allowed = NODE_MASK_NONE;
>>   
>>
>>
>>
>>   	if (!node_demotion)
>>   		return;
>> @@ -511,6 +533,29 @@ static void establish_migration_targets(void)
>>   			}
>>   		} while (1);
>>   	}
>> +	/*
>> +	 * Now build the allowed mask for each node collecting node mask from
>> +	 * all memory tier below it. This allows us to fallback demotion page
>> +	 * allocation to a set of nodes that is closer the above selected
>> +	 * perferred node.
>> +	 */
>> +	list_for_each_entry(memtier, &memory_tiers, list)
>> +		nodes_or(allowed, allowed, memtier->nodelist);
>> +	/*
>> +	 * Removes nodes not yet in N_MEMORY.
>> +	 */
>> +	nodes_and(allowed, node_states[N_MEMORY], allowed);
>> +
>> +	list_for_each_entry(memtier, &memory_tiers, list) {
>> +		/*
>> +		 * Keep removing current tier from allowed nodes,
>> +		 * This will remove all nodes in current and above
>> +		 * memory tier from the allowed mask.
>> +		 */
>> +		nodes_andnot(allowed, allowed, memtier->nodelist);
>> +		for_each_node_mask(node, memtier->nodelist)
>> +			node_demotion[node].allowed = allowed;
>> +	}
>>   }
>>   
>>
>>
>>
>>   /*
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 3a8f78277f99..b0792d838efb 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1460,19 +1460,32 @@ static void folio_check_dirty_writeback(struct folio *folio,
>>   		mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
>>   }
>>   
>>
>>
>>
>> -static struct page *alloc_demote_page(struct page *page, unsigned long node)
>> +static struct page *alloc_demote_page(struct page *page, unsigned long private)
>>   {
>> -	struct migration_target_control mtc = {
>> -		/*
>> -		 * Allocate from 'node', or fail quickly and quietly.
>> -		 * When this happens, 'page' will likely just be discarded
>> -		 * instead of migrated.
>> -		 */
>> -		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>> -			    __GFP_THISNODE  | __GFP_NOWARN |
>> -			    __GFP_NOMEMALLOC | GFP_NOWAIT,
>> -		.nid = node
>> -	};
>> +	struct page *target_page;
>> +	nodemask_t *allowed_mask;
>> +	struct migration_target_control *mtc;
>> +
>> +	mtc = (struct migration_target_control *)private;
>> +
>> +	allowed_mask = mtc->nmask;
>> +	/*
>> +	 * make sure we allocate from the target node first also trying to
>> +	 * reclaim pages from the target node via kswapd if we are low on
>> +	 * free memory on target node. If we don't do this and if we have low
>> +	 * free memory on the target memtier, we would start allocating pages
>> +	 * from higher memory tiers without even forcing a demotion of cold
>> +	 * pages from the target memtier. This can result in the kernel placing
>> +	 * hotpages in higher memory tiers.
>> +	 */
>> +	mtc->nmask = NULL;
>> +	mtc->gfp_mask |= __GFP_THISNODE;
>> +	target_page = alloc_migration_target(page, (unsigned long)&mtc);
>> +	if (target_page)
>> +		return target_page;
>> +
>> +	mtc->gfp_mask &= ~__GFP_THISNODE;
>> +	mtc->nmask = allowed_mask;
>>   
>>
>>
>>
>>   	return alloc_migration_target(page, (unsigned long)&mtc);
>>   }
>> @@ -1487,6 +1500,19 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
>>   {
>>   	int target_nid = next_demotion_node(pgdat->node_id);
>>   	unsigned int nr_succeeded;
>> +	nodemask_t allowed_mask;
>> +
>> +	struct migration_target_control mtc = {
>> +		/*
>> +		 * Allocate from 'node', or fail quickly and quietly.
>> +		 * When this happens, 'page' will likely just be discarded
>> +		 * instead of migrated.
>> +		 */
>> +		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) | __GFP_NOWARN |
>> +			__GFP_NOMEMALLOC | GFP_NOWAIT,
>> +		.nid = target_nid,
>> +		.nmask = &allowed_mask
>> +	};
>>   
>>
>>
>>
>>   	if (list_empty(demote_pages))
>>   		return 0;
>> @@ -1494,10 +1520,12 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
>>   	if (target_nid == NUMA_NO_NODE)
>>   		return 0;
>>   
>>
>>
>>
>> +	node_get_allowed_targets(pgdat->node_id, &allowed_mask);
>> +
>>   	/* Demotion ignores all cpuset and mempolicy settings */
>>   	migrate_pages(demote_pages, alloc_demote_page, NULL,
>> -			    target_nid, MIGRATE_ASYNC, MR_DEMOTION,
>> -			    &nr_succeeded);
>> +		      (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
>> +		      &nr_succeeded);
> 
> Firstly, it addressed my requirement, Thanks!  And, I'd prefer to put
> mtc definition in alloc_demote_page().  Because that makes all page
> allocation logic in one function.  Thus the readability of code is
> better.

The challenge is in allowed_mask computation. That is based on the 
src_node and not target_node.

-aneesh