From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751132AbVLLIcb (ORCPT ); Mon, 12 Dec 2005 03:32:31 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751138AbVLLIcb (ORCPT ); Mon, 12 Dec 2005 03:32:31 -0500 Received: from smtp100.mail.sc5.yahoo.com ([216.136.174.138]:29119 "HELO smtp100.mail.sc5.yahoo.com") by vger.kernel.org with SMTP id S1751132AbVLLIca (ORCPT ); Mon, 12 Dec 2005 03:32:30 -0500 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com.au; h=Received:Message-ID:Date:From:User-Agent:X-Accept-Language:MIME-Version:To:CC:Subject:References:In-Reply-To:Content-Type:Content-Transfer-Encoding; b=RUjWyoNzcoVGV3ZaOObFdgajaGiKa8tA6WyGgnVv0b77oJ2SvWNvDfq9jPGVfQbV0UwVMTecp5eaVOjZs9fXCfFf9v5YTVjC47Lmx4v0C/krnKdr8Bqx1biyZKE0ZRgAVJfq587fBiqrqVmu4uK7a+9M4FhVYes5eWTr9KAUxuU= ; Message-ID: <439D3592.70100@yahoo.com.au> Date: Mon, 12 Dec 2005 19:32:18 +1100 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.12) Gecko/20051007 Debian/1.7.12-1 X-Accept-Language: en MIME-Version: 1.0 To: "Michael S. Tsirkin" CC: Hugh Dickins , Gleb Natapov , Benjamin Herrenschmidt , Petr Vandrovec , Badari Pulavarty , Linux Kernel Mailing List Subject: Re: set_page_dirty vs set_page_dirty_lock References: <439D224A.7080007@yahoo.com.au> <20051212081415.GT14936@mellanox.co.il> In-Reply-To: <20051212081415.GT14936@mellanox.co.il> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Michael S. Tsirkin wrote: > Quoting Nick Piggin : > >>>>>If that works, I can mostly do things directly, >>>>>although I'm still stuck with the problem of an app performing >>>>>a fork + write into the same page while I'm doing DMA there. >>>>> >>>>>I am currently solving this by doing a second get_user_pages after >>>>>DMA is done and comparing the page lists, but this, of course, >>>>>needs a task context ... >>>>> >>>> >>>>Usually we don't care about these kinds of races happening. So long >>>>as it doesn't oops the kernel or hang the hardware, it is up to >>>>userspace not to do stuff like that. >>> >>> >>>Note that I am, even, not necessarily talking about full pages >>>here: an application could be writing to one part of a page >>>while hardware DMAs another part of it. >>>So the app is not necessarily buggy. >>> >> >>Sorry, I might have misunderstdood: what's the race? And how does >>a second get_user_pages solve it? > > > Here's what I have in mind: > > A multithreaded app calls recvmsg(2), (or io_submit with receive request), > passing in a buffer that is not page aligned. > This does get_user_pages on some page and blocks waiting for DMA to complete. > > Another thread calls fork(2), marking the page for copy on write. > After fork, it writes (even 1 byte) into one of the pages that were passed > to recvmsg, possibly even outside the buffer passed to recvmsg. > This triggers a page copy in the parent process. > OK, yeah if a thread in the parent process writes into the buffer, then yes this would leave the copy in the parent AFAIKS. But this is going to do similar weird stuff when racing with copy_to_user with ethernet recvmsg, is it not? (and direct-io and probably others). As such, I don't think it would be something you in particular need to worry about. I guess to solve it, we could either retain mmap_sem for the duration to prevent fork, or try to do something tricky with page_count to determine if we need to do a copy in fork() rather than a COW. -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com