linux-kernel - Re: MPOL_BIND on memory only nodes

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Date:   Thu, 13 Oct 2016 11:24:59 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: MPOL_BIND on memory only nodes

On Wed, Oct 12, 2016 at 03:16:27PM +0200, Michal Hocko wrote:
> On Wed 12-10-16 11:43:37, Michal Hocko wrote:
> > On Wed 12-10-16 14:55:24, Anshuman Khandual wrote:
> [...]
> > > Why we insist on __GFP_THISNODE ?
> > 
> > AFAIU __GFP_THISNODE just overrides the given node to the policy
> > nodemask in case the current node is not part of that node mask. In
> > other words we are ignoring the given node and use what the policy says. 
> > I can see how this can be confusing especially when confronting the
> > documentation:
> > 
> >  * __GFP_THISNODE forces the allocation to be satisified from the requested
> >  *   node with no fallbacks or placement policy enforcements.
> 
> You made me think and look into this deeper. I came to the conclusion
> that this is actually a relict from the past. policy_zonelist is called
> only from 3 places:
> - huge_zonelist - never should do __GFP_THISNODE when going this path
> - alloc_pages_vma - which shouldn't depend on __GFP_THISNODE either
> - alloc_pages_current - which uses default_policy id __GFP_THISNODE is
>   used
> 
> So AFAICS this is essentially a dead code or I am missing something. Mel
> do you remember why we needed it in the past?

I don't recall a specific reason. It was likely due to confusion on my
part at the time on the exact use of __GFP_THISNODE. The expectation is
that flag is not used in fault paths or with policies. It's meant to
enforce node-locality for kernel internal decisions such as the locality
of slab pages and ensuring that a THP collapse from khugepaged is on the
same node.

-- 
Mel Gorman
SUSE Labs