[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <ZTf0mP3fTvhlr/DX@casper.infradead.org>
Date: Tue, 24 Oct 2023 17:45:12 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Hugh Dickins <hughd@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andi Kleen <ak@...ux.intel.com>,
Christoph Lameter <cl@...ux.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
David Hildenbrand <david@...hat.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Yang Shi <shy828301@...il.com>,
Sidhartha Kumar <sidhartha.kumar@...cle.com>,
Vishal Moola <vishal.moola@...il.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Tejun Heo <tj@...nel.org>,
Mel Gorman <mgorman@...hsingularity.net>,
Michal Hocko <mhocko@...e.com>,
"Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH] mempolicy: migration attempt to match interleave nodes:
fix
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@...gle.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.
Powered by blists - more mailing lists