From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41070) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7Vx3-00079N-Lz for qemu-devel@nongnu.org; Tue, 23 Jun 2015 17:41:54 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z7Vwx-0001Mc-JG for qemu-devel@nongnu.org; Tue, 23 Jun 2015 17:41:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42894) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7Vwx-0001La-DA for qemu-devel@nongnu.org; Tue, 23 Jun 2015 17:41:47 -0400 Date: Tue, 23 Jun 2015 23:41:41 +0200 From: Andrea Arcangeli Message-ID: <20150623214141.GB4312@redhat.com> References: <1431624680-20153-1-git-send-email-aarcange@redhat.com> <1431624680-20153-11-git-send-email-aarcange@redhat.com> <5589ACC3.3060401@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5589ACC3.3060401@intel.com> Subject: Re: [Qemu-devel] [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Dave Hansen Cc: "Huangpeng (Peter)" , zhang.zhanghailiang@huawei.com, kvm@vger.kernel.org, Pavel Emelyanov , linux-api@vger.kernel.org, Johannes Weiner , Hugh Dickins , linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, linux-mm@kvack.org, Andres Lagar-Cavilla , Mel Gorman , Paolo Bonzini , "Kirill A. Shutemov" , Andrew Morton , Sanidhya Kashyap , Linus Torvalds , Andy Lutomirski , "Dr. David Alan Gilbert" , Peter Feiner Hi Dave, On Tue, Jun 23, 2015 at 12:00:19PM -0700, Dave Hansen wrote: > Down in userfaultfd_wake_function(), it looks like you intended for a > len=0 to mean "wake all". But the validate_range() that we do from > userspace has a !len check in it, which keeps us from passing a len=0 in > from userspace. > Was that "wake all" for some internal use, or is the check too strict? It's for internal use or userfaultfd_release that has to wake them all (after setting ctx->released) if the uffd is closed. It avoids to enlarge the structure by depending on the invariant that userland cannot pass len=0. If we'd accept len=0 from userland as valid, I'd be safer if it does nothing like in madvise, I doubt we want to expose this non standard kernel internal behavior to userland. > I was trying to use the wake ioctl after an madvise() (as opposed to > filling things in using a userfd copy). madvise will return 0 if len=0, mremap would return -EINVAL if new_len is zero, mmap also returns -EINVAL if len is 0, not all MM syscalls are as permissive as madvise. Can't you pass the same len you pass to madvise to UFFDIO_WAKE (or just skip the call if the madvise len is zero)? Thanks, Andrea