linux-kernel - Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F

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Date:   Fri, 28 May 2021 12:39:54 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>, ying.huang@...el.com
Subject: Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F_LOCAL bit

On Thu, May 27, 2021 at 05:34:56PM +0200, Michal Hocko wrote:
> On Thu 27-05-21 21:34:36, Feng Tang wrote:
> > On Thu, May 27, 2021 at 02:26:24PM +0200, Michal Hocko wrote:
> > > On Thu 27-05-21 20:10:41, Feng Tang wrote:
> > > > On Thu, May 27, 2021 at 10:20:08AM +0200, Michal Hocko wrote:
> > > > > On Wed 26-05-21 13:01:42, Feng Tang wrote:
> > > > > > Now the only remaining case of a real 'local' policy faked by
> > > > > > 'prefer' policy plus MPOL_F_LOCAL bit is:
> > > > > > 
> > > > > > A valid 'prefer' policy with a valid 'preferred' node is 'rebind'
> > > > > > to a nodemask which doesn't contains the 'preferred' node, then it
> > > > > > will handle allocation with 'local' policy.
> > > > > > 
> > > > > > Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the
> > > > > > MPOL_F_LOCAL bit, which could simplify the code much.
> > > > > 
> > > > > As I've pointed out in the reply to the previous patch. It would have
> > > > > been much better if most of the MPOL_F_LOCAL usage was gone by this
> > > > > patch.
> > > > > 
> > > > > I also dislike a new MPOL_F_LOCAL_TEMP. This smells like sneaking the
> > > > > hack back in after you have painstakingly removed it. So this looks like
> > > > > a step backwards to me. I also do not understand why do we need the
> > > > > rebind callback for local policy at all. There is no node mask for local
> > > > > so what is going on here?
> > > > 
> > > > This is the special case 4 for 'perfer' policy with MPOL_F_STATIC_NODES
> > > > flag set, say it prefer node 1, when it is later 'refind' to a new
> > > > nodemask node 2-3, according to current code it will be add the
> > > > MPOL_F_LOCAL bit and performs 'local' policy acctually. And in future
> > > > it is 'rebind' again with a nodemask 1-2, it will be restored back
> > > > to 'prefer' policy with preferred node 1.
> > > 
> > > Honestly I still do not follow the actual problem. 
> > 
> > I was confused too, and don't know the original thought behind it. This
> > case 4 was just imagined by reading the code.
> > 
> > > A preferred node is a
> > > _hint_. If you rebind the task to a different cpuset then why should we
> > > actually care? The allocator will fallback to the closest node according
> > > to the distance metric. Maybe the original code was trying to handle
> > > that in some way but I really do fail to understand that code and I
> > > strongly suspect it is more likely to overengineered rather than backed
> > > by a real usecase. I might be wrong here but then this is an excellent
> > > opportunity to clarify all those subtleties.
> > 
> > From the code, the original special handling may be needed in 3 cases:
> >     get_policy_nodemask()
> >     policy_node()
> >     mempolicy_slab_node()
> > to not return the preset prefer_nid.
> 
> I am sorry but I do not follow. What is actually wrong if the preferred
> node is outside of the cpuset nodemask?

Sorry, I didn't make it clear. With current code logic, it will perform
as 'local' policy, but its mode is kept as 'prefer', so the code still
has these tricky bit checking when these APIs are called for this policy.
I agree with you that these ping-pong rebind() may be over engineering,
so for this case can we just change the policy from 'prefer' to 'local',
and drop the tricky bit manipulation, as the 'prefer' is just a hint,
if these rebind misses the target node, there is no need to stick with
the 'prefer' policy?

Thanks,
Feng