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 2E15D319851 for ; Sat, 19 Sep 2026 18:47:40 +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=1789843662; cv=none; b=mHbqQ9t9wFpIjofNgW7MeFopBgyGo44fyt/vcznX640P3KWgaZhCql02NBvuh6VsLFaCEzNIynih7RL5LfdeEnP4L6U15KNE0qr3oDEVzjIWDVnM1FY/wNvJMhIzHzafnfmKBTtcNVUtK3dLHOKFpKuEFVOwC3hO06GilCp5jNw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789843662; c=relaxed/simple; bh=4qlImrq9ItnesQl8xeHKpKFTXcuwhXTLGbz4iyz3JrA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VG64lTBhg1TgyopGUhaeWDg5lBFE9g5nAxIpkQuZBzvJO2oFQLhpBrN6pnmNZEeEYFjOuQgm7UQWYaE+kT5G34FsmmCdxG5/AKB8BODpDoi7lxqgLH0NasJIgCLb5/tanyPDdjQKeAedCfqStLCWA0S7eyzWajlC0/SbsnOBDwg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WvDXuB2t; 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="WvDXuB2t" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 079071F000FF; Sat, 19 Sep 2026 18:47:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789843660; bh=8qyXPxD8WHiPICoNsVWMXf+1EsR7fxX7tJj8XMR1DcU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=WvDXuB2tiAuNcJOSr7UtZpXxQZRovBEsOYzxDU1hrBkwCpGuHM7I+si0qS30gvOCh J40R+P8OqIVeWnVOJPtu8y0wxucoiBlWWxByDMC46ajsDm4osqMMXfoDRtDgwE/xZa 0XMGrap0LcpPMDdWTEmtIcLSc9+S+Mx1lzmkXQ1ladVleomk+vVkVvMDxLgpqDuw1P 55R006O7elPGojCxCOBQJJCGeO60gWLX8PEyUfXKILIbHnuHeszHM7Q4Qe+GQ0FEsT AaTQLxmNgP9U95oLQ29tt9xDVJgi+MQKRk7bvx7tag3oW/HPSJqcqsnzd4z9IjZyIz fsW6YiK3GB+ag== Message-ID: <2ba1724b-7b10-4b7d-a9f9-c642bc48d3bf@kernel.org> Date: Sat, 19 Sep 2026 20:47:32 +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: disallow raw PMD mappings of the huge zero page To: Lance Yang Cc: kas@kernel.org, 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260919151446.5741-1-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 9/19/26 17:14, Lance Yang wrote: > > On Fri, Sep 18, 2026 at 10:37:27PM +0200, David Hildenbrand (Arm) wrote: >> On 9/18/26 04:52, Lance Yang wrote: >>> >>> >>> No in-tree user inserts the zeropage through vmf_insert_pfn_prot(), so >>> rejecting it there should be fine. >>> >>> Something like: >>> >>> ---8<--- >>> The huge zeropage is reclaimable, but a raw PFN mapping does not pin it. >>> A PMD mapping of huge_zero_pfn can therefore outlive the folio. >> >> Well, just with any other folio that we PFNMAP, the mapping would have to hold a >> reference. mm_get_huge_zero_folio() can be used for that, or just grabbing a >> folio reference (not so nice, but works; GUP uses that, for example, to keep the >> huge zero folio alive). >> >>> >>> No in-tree user needs either mapping, so reject both with VM_FAULT_SIGBUS >>> rather than risk making either shared zero page writable. >> >> That is the better reason. You could also just say that handling shared >> zeropages correctly is more involved (see vm_mixed_ok()), and as there are no >> users, we can just disallow it instead of trying to fix it. > > Ah I see, thanks! obviously, I hadn't fully understood that ... > > How about this instead? > > " > Handling the huge/shared zeropage correctly in vmf_insert_pfn_pmd() > and vmf_insert_pfn_prot() is more involved. We would need to check > whether the VMA allows it and keep the mapping read-only, similar > to the checks in vm_mixed_ok(). > > No in-tree user needs that support, so reject these mappings with > VM_FAULT_SIGBUS rather than complicate the code for now. > " Works for me. -- Cheers, David