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 A94D651DAF6 for ; Fri, 18 Sep 2026 20:37:34 +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=1789763856; cv=none; b=KIPt8sSvW09C+1sSzsNwROxa8WfVD1IFavUcdUV7KRaANvMUAYzcuM0u4qD5nt6jtTr0FageVR4Qf5g9lsGGk3SLfAklIPf0DG2YnXpZlV07/phBLYPuCmIoyXvw5YX455tqdJff0+MvLImNMiMMzx5bOwaohMe76vOyAIip12w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789763856; c=relaxed/simple; bh=xE0xQjQ/iPKm+pax8m4Xy1M+j0dC8fdNT+JtheRoFX4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=G6Il5VzrnnN0tNz3iz4/tmOLszsctOn5YMMsd8fT3bEoS1lxWr45HTcz1oZxq4suBSyi6wGJUQUUpUxLmG/4jZ0/Or4XpFuvR0gNHGM61+ePvwsGkxsNzhxoNCMgDjIf7j368gEjOhrBYk2lk5WbBVT8JUsvt/w6EFTySQiIWQ4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TA8QtEdc; 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="TA8QtEdc" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A53321F000FF; Fri, 18 Sep 2026 20:37:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789763853; bh=3dMJm3ZjQ+ZEzM07+fi70hbZiaOAoeuWuXkc2PtvRng=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TA8QtEdcy2F0tPssdLP6Grp+EXQbWJVgYbobWiS+yfaRhNto37XPC6yydxfJS0ati YN4fk/NnkmXUKF05nS4ureQ7OrBYLmzmgUgdgTt69/E7ZELG1aXjpUGOXfKOm+Q0oF ir70RxL94eq4rHj0GVIOTUZBY07gfXR5p2hoGYUz9y5Sq02iXnae9XyhIWp2dfsiPW nFKd2GW9XpJFGPWvUfTNTkCXp+DAqA5pux1svrvj7hho61/ESPwF/3aNLdtnNp6cNH 5EbQZrfTQV0Duu0JB9hLq+eP5bW5bZk64rYsNJNnRiZZb8Z/DQcH89WMZkJjaqPVLb xXK2kKb8zfwrA== Message-ID: Date: Fri, 18 Sep 2026 22:37:27 +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 , kas@kernel.org 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260918025214.89109-1-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 9/18/26 04:52, Lance Yang wrote: > > On Thu, Sep 17, 2026 at 05:03:09PM +0100, Kiryl Shutsemau wrote: >> On Thu, Sep 17, 2026 at 05:29:48PM +0200, David Hildenbrand (Arm) wrote: >>> >>> Why would the zeropage be a problem in PFNMAP mapping? >> >> Nothing enforces that it is read-only. > > 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. > > Link: https://lore.kernel.org/linux-mm/f76beaf8-351a-4b22-b362-a945a5e1af6a@kernel.org/ > Suggested-by: Kiryl Shutsemau > Suggested-by: David Hildenbrand > Signed-off-by: Lance Yang > --- -- Cheers, David