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

[Ying Huang]

On Fri, 2022-06-03 at 20:39 +0530, Aneesh Kumar K V wrote:
> On 6/2/22 1:05 PM, Ying Huang wrote:
> > On Fri, 2022-05-27 at 17:55 +0530, Aneesh Kumar K.V wrote:
> > > From: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
> > > 
> > > currently, a higher tier node can only be demoted to selected
> > > nodes on the next lower tier as defined by the demotion path,
> > > not any other node from any lower tier.  This strict, hard-coded
> > > demotion order does not work in all use cases (e.g. some use cases
> > > may want to allow cross-socket demotion to another node in the same
> > > demotion tier as a fallback when the preferred demotion node is out
> > > of space). This demotion order is also inconsistent with the page
> > > allocation fallback order when all the nodes in a higher tier are
> > > out of space: The page allocation can fall back to any node from any
> > > lower tier, whereas the demotion order doesn't allow that currently.
> > > 
> > > This patch adds support to get all the allowed demotion targets mask
> > > for node, also demote_page_list() function is modified to utilize this
> > > allowed node mask by filling it in migration_target_control structure
> > > before passing it to migrate_pages().
> > 
> 
> ...
> 
> > >    * Take pages on @demote_list and attempt to demote them to
> > >    * another node.  Pages which are not demoted are left on
> > > @@ -1481,6 +1464,19 @@ 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
> > > +	};
> > 
> > IMHO, we should try to allocate from preferred node firstly (which will
> > kick kswapd of the preferred node if necessary).  If failed, we will
> > fallback to all allowed node.
> > 
> > As we discussed as follows,
> > 
> > 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.

> I guess we can do this as part of the change that is going to 
> introduce the usage of memory policy for the allocation?

Like the memory allocation policy, the default policy should be local
preferred.  We shouldn't force users to use explicit memory policy for
that.

And the added code isn't complex.

Best Regards,
Huang, Ying