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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Jbt9Kafj; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 96134606A324 for ; Mon, 2 Nov 2026 09:07:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Jbt9Kafj Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4E1431F000C1; Mon, 2 Nov 2026 09:07:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793610470; bh=5S5t7qJTf2rCr/p9zCfI7+knHjLhu44+KFJNlFtVB/M=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Jbt9KafjnzhlsJBoEzqWcGNhLY15fXOKREOH+XxGfPFAXJV5Nl3LP/yorfF1n3eSo jOfN7yFcCZxKrIch/i1kHP834YdiLfoRoF3wnj7wJ09ecNRBCBuhVPoG+sSodqh/w/ JCuqvoIJ7jCOkh5kgl/A9M0E7lyhcUqHdyryaQIjkS0cfLUU/bNyeJ9ac2pX4fB0sm YZAHm8NFmWWSJDUU8xXvuot8o6xDcCO4VoM08fOhqcg5/YeT8gnvZBjlLEYVKr4Shm pSYtA/2EC/pJl1ezomF/o0w0vD2am+4wmDP7HtOb8ef5Y8t7FQnEWMXQ+BPvo5wf36 vhVJgeOUg3x/g== Message-ID: <4806f76f-1fd3-4554-8656-86f4802785fc@kernel.org> Date: Mon, 2 Nov 2026 10:07:44 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: fix softlockup in do_pages_stat() with huge page arrays To: Andrew Morton Cc: Qi Xi , linux-mm@kvack.org, Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Qiliang Yuan , linux-kernel@vger.kernel.org, wangkefeng.wang@huawei.com, sunnanyong@huawei.com, xiqi2@huawei.com References: <20261031033330.3416326-1-xiqi5703@163.com> <76579248-64db-47bf-b8d0-4eb3ca556f5a@kernel.org> <20261031094253.d28f484e7e27de469cfa76d2@linux-foundation.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/31/26 17:42, Andrew Morton wrote: > On Sat, 31 Oct 2026 11:52:30 +0100 "David Hildenbrand (Arm)" wrote: > >> On 10/31/26 04:33, Qi Xi wrote: >>> From: Qi Xi >>> >>> do_pages_stat() walks the user page array in 16-page chunks but never >>> reschedules, so a huge nr_pages can hold a CPU past the watchdog >>> threshold on non-preemptible kernels. Add cond_resched() to the loop so >>> long move_pages() node-query calls yield periodically. >>> >>> With KASAN and function tracing enabled, a move_pages() node query >>> (nodes == NULL) over 10 million page entries (~40GB) can trigger the >>> softlockup on a PREEMPT_NONE kernel. >>> >>> Fixes: 80bba1290ab5 ("mm: no get_user/put_user while holding mmap_sem in do_pages_stat?") >>> Signed-off-by: Qi Xi >>> --- >>> mm/migrate.c | 1 + >>> 1 file changed, 1 insertion(+) >>> >>> diff --git a/mm/migrate.c b/mm/migrate.c >>> index 7bdcdb57652f..6c2371020cc5 100644 >>> --- a/mm/migrate.c >>> +++ b/mm/migrate.c >>> @@ -2545,6 +2545,7 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages, >>> >>> chunk_offset += chunk_nr; >>> nr_pages -= chunk_nr; >>> + cond_resched(); >>> } >>> return nr_pages ? -EFAULT : 0; >>> } >> >> I though we wanted to stop throwing in cond_resched() at this point? > > It's almost unneeded in current mainline kernels. PREEMPT_NONE can > still be selected by > > All supported Alpha configurations > > All supported Hexagon configurations > > Non-ColdFire configurations (classic m68k) > > However 99% of the world are on 5.x and 6.x kernels and many of those > machines use PREEMPT_NONE. > > > IOW, cond_resched() is not really needed in mainline, needed in > -stable. But where did the submitter reproduce this? Why should we even care reading the above? I'm all confused now. -- Cheers, David