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

[Ying Huang]

On Mon, 2022-06-06 at 11:51 +0530, Aneesh Kumar K.V wrote:
> Ying Huang <ying.huang@xxxxxxxxx> writes:
> 
> .....
> 
> > > > > 
> > > > > > https://lore.kernel.org/lkml/69f2d063a15f8c4afb4688af7b7890f32af55391.camel@xxxxxxxxx/
> > > > > > 
> > > > > > 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@xxxxxxxxxxxxxx/
> > 
> > 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.

Best Regards,
Huang, Ying

>  	if (current_is_kswapd())
>  		__count_vm_events(PGDEMOTE_KSWAPD, nr_succeeded);