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Date:   Tue, 24 Oct 2023 17:45:12 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Christoph Lameter <cl@linux.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        Vishal Moola <vishal.moola@gmail.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>,
        "Huang, Ying" <ying.huang@intel.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] mempolicy: migration attempt to match interleave nodes:
 fix
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On Tue, Oct 24, 2023 at 09:32:44AM -0700, Hugh Dickins wrote:
> On Tue, 24 Oct 2023, Liam R. Howlett wrote:
> 
> > * Hugh Dickins <hughd@google.com> [231024 02:50]:
> > > mm-unstable commit edd33b8807a1 ("mempolicy: migration attempt to match
> > > interleave nodes") added a second vma_iter search to do_mbind(), to
> > > determine the interleave index to be used in the MPOL_INTERLEAVE case.
> > > 
> > > But sadly it added it just after the mmap_write_unlock(), leaving this
> > > new VMA search unprotected: and so syzbot reports suspicious RCU usage
> > > from lib/maple_tree.c:856.
> > > 
> > > This could be fixed with an rcu_read_lock/unlock() pair (per Liam);
> > > but since we have been relying on the mmap_lock up to this point, it's
> > > slightly better to extend it over the new search too, for a well-defined
> > > result consistent with the policy this mbind() is establishing (rather
> > > than whatever might follow once the mmap_lock is dropped).
> > 
> > Would downgrading the lock work?  It would avoid the potential writing
> > issue and should still satisfy lockdep.
> 
> Downgrading the lock would work, but it would be a pointless complication.

I tend to agree.  It's also becoming far less important these days
with the vast majority of page faults handled under the per-VMA lock.
We might be able to turn it into a mutex instead of an rwsem without
seeing a noticable drop-off in performance.  Not volunteering to try this.