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From: Hugh Dickins <hughd@google.com>
To: "Liam R. Howlett" <Liam.Howlett@oracle.com>,
	Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andi Kleen <ak@linux.intel.com>, Christoph Lameter <cl@linux.com>,
	Matthew Wilcox <willy@infradead.org>,
	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
Date: Tue, 24 Oct 2023 09:32:44 -0700 (PDT)	[thread overview]
Message-ID: <5b570bcd-e0fb-4473-7cfc-81b8b53d7712@google.com> (raw)
In-Reply-To: <20231024151827.i7e2f6cheet4k5h5@revolver>

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.

The "second vma_iter search" is not a lengthy operation (normally it just
checks pgoff,start,end of the first VMA and immediately breaks out; in
worst case it just makes that check on each VMA involved: it doesn't get
into splits or merges or pte scans), we already have mmap_lock, yes it's
only needed for read during that scani, but it's not worth playing with.

Whereas migrating an indefinite number of pages, with all the allocating
and unmapping and copying and remapping involved, really is something we
prefer not to hold mmap_lock across.

Hugh

  reply	other threads:[~2023-10-24 16:33 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-10-03  9:12 [PATCH v2 00/12] mempolicy: cleanups leading to NUMA mpol without vma Hugh Dickins
2023-10-03  9:15 ` [PATCH v2 01/12] hugetlbfs: drop shared NUMA mempolicy pretence Hugh Dickins
2023-10-03  9:16 ` [PATCH v2 02/12] kernfs: drop shared NUMA mempolicy hooks Hugh Dickins
2023-10-03  9:17 ` [PATCH v2 03/12] mempolicy: fix migrate_pages(2) syscall return nr_failed Hugh Dickins
2023-10-07  7:27   ` Huang, Ying
2023-10-03  9:19 ` [PATCH v2 04/12] mempolicy trivia: delete those ancient pr_debug()s Hugh Dickins
2023-10-03  9:20 ` [PATCH v2 05/12] mempolicy trivia: slightly more consistent naming Hugh Dickins
2023-10-03  9:21 ` [PATCH v2 06/12] mempolicy trivia: use pgoff_t in shared mempolicy tree Hugh Dickins
2023-10-03  9:22 ` [PATCH v2 07/12] mempolicy: mpol_shared_policy_init() without pseudo-vma Hugh Dickins
2023-10-03  9:24 ` [PATCH v2 08/12] mempolicy: remove confusing MPOL_MF_LAZY dead code Hugh Dickins
2023-10-03 22:28   ` Yang Shi
2023-10-03  9:25 ` [PATCH v2 09/12] mm: add page_rmappable_folio() wrapper Hugh Dickins
2023-10-03  9:26 ` [PATCH v2 10/12] mempolicy: alloc_pages_mpol() for NUMA policy without vma Hugh Dickins
2023-10-19 20:39   ` [PATCH v3 " Hugh Dickins
2023-10-23 16:53     ` domenico cerasuolo
2023-10-23 17:53       ` Andrew Morton
2023-10-23 18:10         ` domenico cerasuolo
2023-10-23 19:05           ` Johannes Weiner
2023-10-23 19:48             ` Hugh Dickins
2023-10-24  6:44             ` [PATCH] mempolicy: alloc_pages_mpol() for NUMA policy without vma: fix Hugh Dickins
2023-10-24  8:17               ` kernel test robot
2023-10-24 15:56                 ` Hugh Dickins
2023-10-24 16:09               ` [PATCH v2] " Hugh Dickins
2023-10-23 18:34     ` [PATCH v3 10/12] mempolicy: alloc_pages_mpol() for NUMA policy without vma Zi Yan
2023-10-23 21:10       ` Hugh Dickins
2023-10-23 21:13         ` Zi Yan
2023-10-03  9:27 ` [PATCH v2 11/12] mempolicy: mmap_lock is not needed while migrating folios Hugh Dickins
2023-10-03  9:29 ` [PATCH v2 12/12] mempolicy: migration attempt to match interleave nodes Hugh Dickins
2023-10-24  6:50   ` [PATCH] mempolicy: migration attempt to match interleave nodes: fix Hugh Dickins
2023-10-24 15:18     ` Liam R. Howlett
2023-10-24 16:32       ` Hugh Dickins [this message]
2023-10-24 16:45         ` Matthew Wilcox

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