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

Ying Huang
2022-06-06T07:43:28Z

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.

Best Regards,
Huang, Ying

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