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Message-ID: <20231024151827.i7e2f6cheet4k5h5@revolver>
Date: Tue, 24 Oct 2023 11:18:27 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Hugh Dickins <hughd@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Andi Kleen <ak@...ux.intel.com>,
Christoph Lameter <cl@...ux.com>,
Matthew Wilcox <willy@...radead.org>,
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
* 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.
>
> Reported-by: syzbot+79fcba037b6df73756d3@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-mm/000000000000c05f1b0608657fde@google.com/
> Fixes: edd33b8807a1 ("mempolicy: migration attempt to match interleave nodes")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
> mm/mempolicy.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 989293180eb6..5e472e6e0507 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1291,8 +1291,6 @@ static long do_mbind(unsigned long start, unsigned long len,
> }
> }
>
> - mmap_write_unlock(mm);
> -
> if (!err && !list_empty(&pagelist)) {
> /* Convert MPOL_DEFAULT's NULL to task or default policy */
> if (!new) {
> @@ -1334,7 +1332,11 @@ static long do_mbind(unsigned long start, unsigned long len,
> mmpol.ilx -= page->index >> order;
> }
> }
> + }
>
> + mmap_write_unlock(mm);
> +
> + if (!err && !list_empty(&pagelist)) {
> nr_failed |= migrate_pages(&pagelist,
> alloc_migration_target_by_mpol, NULL,
> (unsigned long)&mmpol, MIGRATE_SYNC,
> --
> 2.35.3
>
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