From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0CA4747F3B9; Mon, 14 Sep 2026 14:29:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789396151; cv=none; b=tMqS1qSoINdKLT6ZcLT6jNXzi03TiH4A3u8d2u/Xp8LhCI5szcEmxfS+/tXEx8+af1ez/z6pX2n0Xa7knk1TF2Qpw7m7nbN5KzFOTuMSMdMOYzNR2poxtr64YSeiDnI6l/aH51D3x925ZZDLPYgM8wpVs4txIFJY9WCAGPxF4d0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789396151; c=relaxed/simple; bh=ZPHHFRzBLD+jlX/jFSCNgdyvMTzLe10Z6as8xZPz2PY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=o8NluyHOUbwJuhEvJhpV4Nwh+q7qudxRF1x1cwe903cY7jgkHbWpwLW46E/KVf8hEc6m5CjE4Hao/Fcz0NNb790Wp3jF2UyoTZCwo3tVWEtGZipaCiJR7+yZ8Ypqs2p3gYAl+ZVoghHU84aQQqv12+1/qctLJWjcCGve2bcik0s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=P3Edea9U; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="P3Edea9U" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C88771F000FF; Mon, 14 Sep 2026 14:29:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789396149; bh=+qLblU+3Kc4+/X9TepAPoRZL2UWEHpF8i+gM8rIH7Q8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=P3Edea9UKwT+ny0d7XBx83UgYk05fCB/jMjEhc0B5PYf7aiiBFtQFxjTF3OBL3f0P /KneYLOoTqqD8j4dFYbfF4TjTLgkXMuXhynrDa7m1NcDL8LeHg1HSwM58+IpByfjy9 RV5mGB8ueay6JXN6LVNW4BIiPb+peoqB0VpIkPhcGGIAaqnCTMnHoyiHPnALF13ss+ 9yChybeBbnpgFhhqZfKjnJud3NnMckIL3A8jhGlzmxbh/FXqrGfZrtbwSWYEGjA+Ne VZ6CuMQL5vc3mwZg6fk9tgPsADZuPPM0i6q/aR4gmfQ6/EFjUQ9xuqpqgPwVJ2A+Aw MHPZokLRbC28A== Message-ID: Date: Mon, 14 Sep 2026 16:29:00 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/huge_memory: fix pgtable withdrawal for huge zero PMDs To: Kiryl Shutsemau , Lance Yang Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, ljs@kernel.org, surenb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260912234635.db50397364858aa15f58f4d7@linux-foundation.org> <20260913072312.52111-1-lance.yang@linux.dev> 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 9/14/26 12:59, Kiryl Shutsemau wrote: > On Sun, Sep 13, 2026 at 03:23:12PM +0800, Lance Yang wrote: >> >> On Sat, Sep 12, 2026 at 11:46:35PM -0700, Andrew Morton wrote: >>> >>> >>> That's the sort of thing we'd prefer to avoid. >>> >>> >>> How real is this? Is there a reported-by:? Do you have a reproducer? >> >> Yes, I reproduced it on x86 with a small test module. It sets >> VM_MIXEDMAP | VM_HUGEPAGE and calls vmf_insert_pfn_pmd() with >> huge_zero_pfn, without touching the page tables directly. A full-PMD >> munmap() crashes before the split series[1] as well. > > Ah. So there's no real bug upstream, right? Matches my understanding. > > And I am not sure it is how we want to address this. I don't think we > should allow randomly map huge zero page (and non-huge too). It can be a > security risk if it ever gets exposed writable. Due to some DAX stuff (and others) we have the shared zeropage in VM_MIXEDMAP. And we have some elaborate checks in vm_mixed_zeropage_allowed() to test for that. We should do a similar validation, and ideally just forbid it if not used yet. -- Cheers, David