Re: [RFC PATCH 5/5] mm, page_alloc: Introduce ZONELIST_FALLBACK_SAME_TYPE fallback list

[Michal Hocko]

On Thu 25-04-19 07:43:09, Du, Fan wrote:
> 
> 
> >-----Original Message-----
> >From: Michal Hocko [mailto:mhocko@xxxxxxxxxx]
> >Sent: Thursday, April 25, 2019 2:38 PM
> >To: Du, Fan <fan.du@xxxxxxxxx>
> >Cc: akpm@xxxxxxxxxxxxxxxxxxxx; Wu, Fengguang <fengguang.wu@xxxxxxxxx>;
> >Williams, Dan J <dan.j.williams@xxxxxxxxx>; Hansen, Dave
> ><dave.hansen@xxxxxxxxx>; xishi.qiuxishi@xxxxxxxxxxxxxxx; Huang, Ying
> ><ying.huang@xxxxxxxxx>; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> >Subject: Re: [RFC PATCH 5/5] mm, page_alloc: Introduce
> >ZONELIST_FALLBACK_SAME_TYPE fallback list
> >
> >On Thu 25-04-19 09:21:35, Fan Du wrote:
> >> On system with heterogeneous memory, reasonable fall back lists woul be:
> >> a. No fall back, stick to current running node.
> >> b. Fall back to other nodes of the same type or different type
> >>    e.g. DRAM node 0 -> DRAM node 1 -> PMEM node 2 -> PMEM node 3
> >> c. Fall back to other nodes of the same type only.
> >>    e.g. DRAM node 0 -> DRAM node 1
> >>
> >> a. is already in place, previous patch implement b. providing way to
> >> satisfy memory request as best effort by default. And this patch of
> >> writing build c. to fallback to the same node type when user specify
> >> GFP_SAME_NODE_TYPE only.
> >
> >So an immediate question which should be answered by this changelog. Who
> >is going to use the new gfp flag? Why cannot all allocations without an
> >explicit numa policy fallback to all existing nodes?
> 
> PMEM is good for frequently read accessed page, e.g. page cache(implicit page
> request), or user space data base (explicit page request)
> For now this patch create GFP_SAME_NODE_TYPE for such cases, additional
> Implementation will be followed up.

Then simply configure that NUMA node as movable and you get these
allocations for any movable allocation. I am not really convinced a new
gfp flag is really justified.
-- 
Michal Hocko
SUSE Labs