Re: [RFC 1/6] mm, page_alloc: fix more premature OOM due to race with cpuset update

From: Christoph Lameter <cl@linux.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	cgroups@vger.kernel.org, Li Zefan <lizefan@huawei.com>,
	Michal Hocko <mhocko@kernel.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	David Rientjes <rientjes@google.com>,
	Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Anshuman Khandual <khandual@linux.vnet.ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	linux-api@vger.kernel.org
Subject: Re: [RFC 1/6] mm, page_alloc: fix more premature OOM due to race with cpuset update
Date: Wed, 12 Apr 2017 16:25:32 -0500 (CDT)	[thread overview]
Message-ID: <alpine.DEB.2.20.1704121617040.28335@east.gentwo.org> (raw)
In-Reply-To: <a86ae57a-3efc-6ae5-ddf0-fd64c53c20fa@suse.cz>

On Tue, 11 Apr 2017, Vlastimil Babka wrote:

> > The fallback was only intended for a cpuset on which boundaries are not enforced
> > in critical conditions (softwall). A hardwall cpuset (CS_MEM_HARDWALL)
> > should fail the allocation.
>
> Hmm just to clarify - I'm talking about ignoring the *mempolicy's* nodemask on
> the basis of cpuset having higher priority, while you seem to be talking about
> ignoring a (softwall) cpuset nodemask, right? man set_mempolicy says "... if
> required nodemask contains no nodes that are allowed by the process's current
> cpuset context, the memory  policy reverts to local allocation" which does come
> down to ignoring mempolicy's nodemask.

I am talking of allocating outside of the current allowed nodes
(determined by mempolicy -- MPOL_BIND is the only concern as far as I can
tell -- as well as the current cpuset). One can violate the cpuset if its not
a hardwall but  the MPOL_MBIND node restriction cannot be violated.

Those allocations are also not allowed if the allocation was for a user
space page even if this is a softwall cpuset.

> >> This patch fixes the issue by having __alloc_pages_slowpath() check for empty
> >> intersection of cpuset and ac->nodemask before OOM or allocation failure. If
> >> it's indeed empty, the nodemask is ignored and allocation retried, which mimics
> >> node_zonelist(). This works fine, because almost all callers of
> >
> > Well that would need to be subject to the hardwall flag. Allocation needs
> > to fail for a hardwall cpuset.
>
> They still do, if no hardwall cpuset node can satisfy the allocation with
> mempolicy ignored.

If the memory policy is MPOL_MBIND then allocations outside of the given
nodes should fail. They can violate the cpuset boundaries only if they are
kernel allocations and we are not in a hardwall cpuset.

That was at least my understand when working on this code years ago.