Re: [RFC PATCH] mm/mempolicy: add MPOL_PREFERRED_STRICT memory policy

From: Michal Hocko <mhocko@suse.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>,
	linux-mm@kvack.org, akpm@linux-foundation.org,
	Ben Widawsky <ben.widawsky@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Feng Tang <feng.tang@intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Huang Ying <ying.huang@intel.com>
Subject: Re: [RFC PATCH] mm/mempolicy: add MPOL_PREFERRED_STRICT memory policy
Date: Thu, 14 Oct 2021 11:38:42 +0200	[thread overview]
Message-ID: <YWf6oiXM+L8J7yyW@dhcp22.suse.cz> (raw)
In-Reply-To: <87pms8ymvl.fsf@linux.ibm.com>

On Thu 14-10-21 15:00:22, Aneesh Kumar K.V wrote:
> Michal Hocko <mhocko@suse.com> writes:
> 
> > On Wed 13-10-21 18:53:55, Aneesh Kumar K.V wrote:
> >> On 10/13/21 18:46, Andi Kleen wrote:
> >> > 
> >> > > The difference with MPOL_BIND is the ability to specify a preferred node
> >> > > which is the first node in the nodemask argument passed.
> >> > 
> >> > That's always the one with the lowest number. Isn't that quite limiting
> >> > in practice?
> >> > 
> >> > It seems if you really want to do that you would need another argument.
> >> > 
> >> Yes. But that would make it a new syscall. Should we do that?
> >
> > Yes, I do not see any reasonable to cram this into the existing syscall.
> > I am not yet sure what the syscall should look like though. I can see
> > two usecases, one of the is a very specific node allocation fallback
> > order requirement and another one is preferrence for a cpu less node
> > over other nodes. Both are slightly different.
> 
> How about
> 
> SYSCALL_DEFINE5(preferred_mbind, unsigned long, start, unsigned long, len,
> 		unsigned long, preferred_node, const unsigned long __user *, nmask,
> 		unsigned long, maxnode)
> {
> 	return kernel_mbind(start, len, MPOL_PREFERRED_STRICT, preferred_node,
> 			    nmask, maxnode, 0);
> }

Semantic? How does it interact with MPOL_PREFERRED_MANY, MPOL_BIND and
other others?

Besides that it would be really great to finish the discussion about the
usecase before suggesting a new userspace API.

-- 
Michal Hocko
SUSE Labs