From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create() Date: Tue, 17 Jun 2014 09:41:22 -0700 Message-ID: References: <1402655819-14325-1-git-send-email-dh.herrmann@gmail.com> <53A01049.6020502@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 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 To: David Herrmann Return-path: In-Reply-To: Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org 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? --Andy -- 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