linux-kernel - Re: MPOL_BIND on memory only nodes

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Date:   Thu, 13 Oct 2016 14:38:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...e.de>
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 Thu 13-10-16 11:24:59, Mel Gorman wrote:
> 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.

This is my understanding as well. Thanks for double checking. I will
send a proper patch (it will even compile as a bonus point ;).
-- 
Michal Hocko
SUSE Labs