From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Herrmann Subject: Re: [PATCH 2/6] shm: add sealing API Date: Fri, 11 Apr 2014 23:31:27 +0200 Message-ID: References: <53470E26.2030306@cybernetics.com> <5347451C.4060106@amacapital.net> <5347F188.10408@cybernetics.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-fsdevel , linux-mm , linux-kernel , "dri-devel@lists.freedesktop.org" , Andy Lutomirski To: Tony Battersby Return-path: In-Reply-To: <5347F188.10408@cybernetics.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" List-Id: linux-fsdevel.vger.kernel.org Hi On Fri, Apr 11, 2014 at 3:43 PM, Tony Battersby wrote: > Exactly. For O_DIRECT, that would be the call to get_user_pages_fast() > from dio_refill_pages() in fs/direct-io.c, which is ultimately called > from blkdev_direct_IO(). If you drop mmap_sem after pinning a page without taking a write-ref, you break i_mmap_writable / VM_DENYWRITE. In memfd I rely on i_mmap_writable to work, same thing is done by exec() (and the old, now disabled, MAP_DENYWRITE). I don't know whether I should care. I mean, everyone pinning pages and writing to it without holding the mmap_sem has to take a write-ref for each page or it breaks i_mmap_writable. So this seems to be a bug in direct-IO, not in anyone relying on it, right? Thanks David