From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 820DCCCA48B for ; Mon, 25 Jul 2022 13:51:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235120AbiGYNvd (ORCPT ); Mon, 25 Jul 2022 09:51:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36044 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229514AbiGYNv3 (ORCPT ); Mon, 25 Jul 2022 09:51:29 -0400 Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B70D815736; Mon, 25 Jul 2022 06:51:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1658757088; x=1690293088; h=date:from:to:cc:subject:message-id:reply-to:references: mime-version:in-reply-to; bh=oTuDloTSJTh/mR+M/XiSHbTOyhEzShF3fA4W384Cvww=; b=ivLkHg85qkQ5t/atKgbyZ9VKg1uvgvgQczzgMsc/ElXWTv2upvBLzMN3 PHhDcpCZQxAl8W+r0eQoHXExx5hHPXkNxxVNZQ1vVWocpnsEqtl17AFHs aoputNef/ssbpXvJ0vq7DR0MKI3SmAsje+XQgcrshZafGtiwChmFLCRsi brf5GGj6FscuKCij9QpuiGhVEUu2NlE2c7pu6iy3/sUkB2QiJ9PVue7Ie 64ZtWLwTT28tpEQi3J8c+Jd8bRWjHvPkNdBAIIMKTTSqBKgxAOtnSPZqB dwdO3QKjVrRNFVZgiVBeBuoaHxNcsbVMPGD4tndAdHRN8ehM+wVovVYR3 g==; X-IronPort-AV: E=McAfee;i="6400,9594,10418"; a="286459637" X-IronPort-AV: E=Sophos;i="5.93,193,1654585200"; d="scan'208";a="286459637" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Jul 2022 06:51:28 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.93,193,1654585200"; d="scan'208";a="627458071" Received: from chaop.bj.intel.com (HELO localhost) ([10.240.193.75]) by orsmga008.jf.intel.com with ESMTP; 25 Jul 2022 06:51:16 -0700 Date: Mon, 25 Jul 2022 21:46:28 +0800 From: Chao Peng To: Sean Christopherson Cc: David Hildenbrand , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org, linux-kselftest@vger.kernel.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Shuah Khan , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Vishal Annapurve , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com, Muchun Song Subject: Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd Message-ID: <20220725134628.GC304216@chaop.bj.intel.com> Reply-To: Chao Peng References: <20220706082016.2603916-1-chao.p.peng@linux.intel.com> <20220706082016.2603916-2-chao.p.peng@linux.intel.com> <39067d09-b32b-23a6-ae0e-00ac2fe0466c@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 21, 2022 at 03:05:09PM +0000, Sean Christopherson wrote: > On Thu, Jul 21, 2022, David Hildenbrand wrote: > > On 21.07.22 11:44, David Hildenbrand wrote: > > > On 06.07.22 10:20, Chao Peng wrote: > > >> Normally, a write to unallocated space of a file or the hole of a sparse > > >> file automatically causes space allocation, for memfd, this equals to > > >> memory allocation. This new seal prevents such automatically allocating, > > >> either this is from a direct write() or a write on the previously > > >> mmap-ed area. The seal does not prevent fallocate() so an explicit > > >> fallocate() can still cause allocating and can be used to reserve > > >> memory. > > >> > > >> This is used to prevent unintentional allocation from userspace on a > > >> stray or careless write and any intentional allocation should use an > > >> explicit fallocate(). One of the main usecases is to avoid memory double > > >> allocation for confidential computing usage where we use two memfds to > > >> back guest memory and at a single point only one memfd is alive and we > > >> want to prevent memory allocation for the other memfd which may have > > >> been mmap-ed previously. More discussion can be found at: > > >> > > >> https://lkml.org/lkml/2022/6/14/1255 > > >> > > >> Suggested-by: Sean Christopherson > > >> Signed-off-by: Chao Peng > > >> --- > > >> include/uapi/linux/fcntl.h | 1 + > > >> mm/memfd.c | 3 ++- > > >> mm/shmem.c | 16 ++++++++++++++-- > > >> 3 files changed, 17 insertions(+), 3 deletions(-) > > >> > > >> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h > > >> index 2f86b2ad6d7e..98bdabc8e309 100644 > > >> --- a/include/uapi/linux/fcntl.h > > >> +++ b/include/uapi/linux/fcntl.h > > >> @@ -43,6 +43,7 @@ > > >> #define F_SEAL_GROW 0x0004 /* prevent file from growing */ > > >> #define F_SEAL_WRITE 0x0008 /* prevent writes */ > > >> #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */ > > >> +#define F_SEAL_AUTO_ALLOCATE 0x0020 /* prevent allocation for writes */ > > > > > > Why only "on writes" and not "on reads". IIRC, shmem doesn't support the > > > shared zeropage, so you'll simply allocate a new page via read() or on > > > read faults. > > > > Correction: on read() we don't allocate a fresh page. But on read faults > > we would. So this comment here needs clarification. > > Not just the comment, the code too. The intent of F_SEAL_AUTO_ALLOCATE is very > much to block _all_ implicit allocations (or maybe just fault-based allocations > if "implicit" is too broad of a description). So maybe still your initial suggestion F_SEAL_FAULT_ALLOCATIONS? One reason I don't like it is the write() ioctl also cause allocation and we want to prevent it. Chao