Re: [RFC][PATCH 8/8] mm/numa: new reclaim mode to enable reclaim-based migration

[Huang\, Ying]

Hi, Yang,

Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:

>>> diff -puN mm/vmscan.c~enable-numa-demotion mm/vmscan.c
>>> --- a/mm/vmscan.c~enable-numa-demotion	2020-06-29 16:35:01.017312549 -0700
>>> +++ b/mm/vmscan.c	2020-06-29 16:35:01.023312549 -0700
>>> @@ -4165,9 +4165,10 @@ int node_reclaim_mode __read_mostly;
>>>    * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
>>>    * ABI.  New bits are OK, but existing bits can never change.
>>>    */
>>> -#define RECLAIM_RSVD  (1<<0)	/* (currently ignored/unused) */
>>> -#define RECLAIM_WRITE (1<<1)	/* Writeout pages during reclaim */
>>> -#define RECLAIM_UNMAP (1<<2)	/* Unmap pages during reclaim */
>>> +#define RECLAIM_RSVD	(1<<0)	/* (currently ignored/unused) */
>>> +#define RECLAIM_WRITE	(1<<1)	/* Writeout pages during reclaim */
>>> +#define RECLAIM_UNMAP	(1<<2)	/* Unmap pages during reclaim */
>>> +#define RECLAIM_MIGRATE	(1<<3)	/* Migrate pages during reclaim */
>>>     /*
>>>    * Priority for NODE_RECLAIM. This determines the fraction of pages
>> I found that RECLAIM_MIGRATE is defined but never referenced in the
>> patch.
>>
>> If my understanding of the code were correct, shrink_do_demote_mapping()
>> is called by shrink_page_list(), which is used by kswapd and direct
>> reclaim.  So as long as the persistent memory node is onlined,
>> reclaim-based migration will be enabled regardless of node reclaim mode.
>
> It looks so according to the code. But the intention of a new node
> reclaim mode is to do migration on reclaim *only when* the
> RECLAIM_MODE is enabled by the users.
>
> It looks the patch just clear the migration target node masks if the
> memory is offlined.
>
> So, I'm supposed you need check if node_reclaim is enabled before
> doing migration in shrink_page_list() and also need make node reclaim
> to adopt the new mode.

But why shouldn't we migrate in kswapd and direct reclaim?  I think that
we may need a way to control it, but shouldn't disable it
unconditionally.

> Please refer to
> https://lore.kernel.org/linux-mm/1560468577-101178-6-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx/
>

Best Regards,
Huang, Ying