From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f178.google.com (mail-ig0-f178.google.com [209.85.213.178]) by kanga.kvack.org (Postfix) with ESMTP id A4AFC6B003A for ; Tue, 17 Jun 2014 12:51:19 -0400 (EDT) Received: by mail-ig0-f178.google.com with SMTP id hn18so4415790igb.17 for ; Tue, 17 Jun 2014 09:51:19 -0700 (PDT) Received: from mail-ie0-x22c.google.com (mail-ie0-x22c.google.com [2607:f8b0:4001:c03::22c]) by mx.google.com with ESMTPS id b11si11617402icj.83.2014.06.17.09.51.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 17 Jun 2014 09:51:19 -0700 (PDT) Received: by mail-ie0-f172.google.com with SMTP id lx4so6588082iec.3 for ; Tue, 17 Jun 2014 09:51:18 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <1402655819-14325-1-git-send-email-dh.herrmann@gmail.com> <53A01049.6020502@redhat.com> Date: Tue, 17 Jun 2014 18:51:18 +0200 Message-ID: Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create() From: David Herrmann Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: "linux-mm@kvack.org" , Greg KH , Florian Weimer , Hugh Dickins , "linux-kernel@vger.kernel.org" , Lennart Poettering , Andrew Morton , Linux API , Michael Kerrisk , Kay Sievers , John Stultz , Linus Torvalds , Daniel Mack , Ryan Lortie , Linux FS Devel , Tony Battersby Hi On Tue, Jun 17, 2014 at 6:41 PM, Andy Lutomirski wrote: > On Tue, Jun 17, 2014 at 9:36 AM, David Herrmann wrote: >> Hi >> >> On Tue, Jun 17, 2014 at 6:20 PM, Andy Lutomirski wrote: >>> Can you summarize why holes can't be reliably backed by the zero page? >> >> To answer this, I will quote Hugh from "PATCH v2 1/3": >> >>> We do already use the ZERO_PAGE instead of allocating when it's a >>> simple read; and on the face of it, we could extend that to mmap >>> once the file is sealed. But I am rather afraid to do so - for >>> many years there was an mmap /dev/zero case which did that, but >>> it was an easily forgotten case which caught us out at least >>> once, so I'm reluctant to reintroduce it now for sealing. >>> >>> Anyway, I don't expect you to resolve the issue of sealed holes: >>> that's very much my territory, to give you support on. >> >> Holes can be avoided with a simple fallocate(). I don't understand why >> I should make SEAL_WRITE do the fallocate for the caller. During the >> discussion of memfd_create() I was told to drop the "size" parameter, >> because it is redundant. I don't see how this implicit fallocate() >> does not fall into the same category? >> > > I'm really confused now. > > If I SEAL_WRITE a file, and then I mmap it PROT_READ, and then I read > it, is that a "simple read"? If so, doesn't that mean that there's no > problem? I assumed Hugh was talking about read(). So no, this is not about memory-reads on mmap()ed regions. Looking at shmem_file_read_iter() I can see a ZERO_PAGE(0) call in case shmem_getpage_gfp(SGP_READ) tells us there's a hole. I cannot see anything like that in the mmap_region() and shmem_fault() paths. Thanks David -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org