From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id 7D7106B0082 for ; Tue, 14 Jun 2011 11:43:08 -0400 (EDT) Date: Tue, 14 Jun 2011 17:40:52 +0200 From: Oleg Nesterov Subject: Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement. Message-ID: <20110614154052.GA22082@redhat.com> References: <20110607125804.28590.92092.sendpatchset@localhost6.localdomain6> <20110607125835.28590.25476.sendpatchset@localhost6.localdomain6> <20110613170020.GA27137@redhat.com> <1308056477.19856.35.camel@twins> <20110614142710.GB5139@redhat.com> <1308064053.19856.75.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1308064053.19856.75.camel@twins> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Srikar Dronamraju , Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Masami Hiramatsu , Hugh Dickins , Christoph Hellwig , Andi Kleen , Thomas Gleixner , Jonathan Corbet , Andrew Morton , Jim Keniston , Roland McGrath , Ananth N Mavinakayanahalli , LKML On 06/14, Peter Zijlstra wrote: > > On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote: > > On 06/14, Peter Zijlstra wrote: > > > > > > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote: > > > > > > > > Also. This is called under down_read(mmap_sem), can't we race with > > > > access_process_vm() modifying the same memory? > > > > > > Shouldn't matter COW and similar things are serialized using the pte > > > lock. > > > > Yes, but afaics this doesn't matter. Suppose that write_opcode() is > > called when access_process_vm() does copy_to_user_page(). We can cow > > the page before memcpy() completes. > > access_process_vm() will end up doing a FOLL_WRITE itself when > copy_to_user_page() is called since write=1 in that case. > > At that point we have a COW-race, someone wins, but the other will then > return the same page. > > At this point further PTRACE pokes can indeed race with the memcpy in > write_opcode(). Currently it can't, write_opcode() does another cow. But that cow can, and this is the same, yes. > A possible fix would be to lock_page() around > copy_to_user_page() (its already done in set_page_dirty_lock(), so > pulling it out shouldn't matter much). Yes, or write_opcode() could take mmap_sem for writing as Srikar suggests. But do we really care? Whatever we do we can race with the other updates to this memory. Say, someone can write to vma->vm_file. Oleg. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org