From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934984Ab1JEQ07 (ORCPT ); Wed, 5 Oct 2011 12:26:59 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:40826 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934476Ab1JEQ06 (ORCPT ); Wed, 5 Oct 2011 12:26:58 -0400 Date: Wed, 5 Oct 2011 21:39:34 +0530 From: Srikar Dronamraju To: Oleg Nesterov Cc: Peter Zijlstra , Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Masami Hiramatsu , Hugh Dickins , Christoph Hellwig , Ananth N Mavinakayanahalli , Thomas Gleixner , Jonathan Corbet , LKML , Jim Keniston , Roland McGrath , Andi Kleen , Andrew Morton Subject: Re: [PATCH v5 3.1.0-rc4-tip 5/26] Uprobes: copy of the original instruction. Message-ID: <20111005160934.GC806@linux.vnet.ibm.com> Reply-To: Srikar Dronamraju References: <20110920115938.25326.93059.sendpatchset@srdronam.in.ibm.com> <20110920120057.25326.63780.sendpatchset@srdronam.in.ibm.com> <20111003162905.GA3752@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20111003162905.GA3752@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Oleg Nesterov [2011-10-03 18:29:05]: > On 09/20, Srikar Dronamraju wrote: > > > > +static int __copy_insn(struct address_space *mapping, > > + struct vm_area_struct *vma, char *insn, > > + unsigned long nbytes, unsigned long offset) > > +{ > > + struct file *filp = vma->vm_file; > > + struct page *page; > > + void *vaddr; > > + unsigned long off1; > > + unsigned long idx; > > + > > + if (!filp) > > + return -EINVAL; > > + > > + idx = (unsigned long) (offset >> PAGE_CACHE_SHIFT); > > + off1 = offset &= ~PAGE_MASK; > > + > > + /* > > + * Ensure that the page that has the original instruction is > > + * populated and in page-cache. > > + */ > > Hmm. But how we can ensure? > > > + page_cache_sync_readahead(mapping, &filp->f_ra, filp, idx, 1); > > This schedules the i/o, > > > + page = grab_cache_page(mapping, idx); > > This finds/locks the page in the page-cache, > > > + if (!page) > > + return -ENOMEM; > > + > > + vaddr = kmap_atomic(page); > > + memcpy(insn, vaddr + off1, nbytes); > > What if this page is not PageUptodate() ? > > Somehow this assumes that the i/o was already completed, I don't > understand this. > > But I am starting to think I simply do not understand this change. > To the point, I do not underestand why do we need copy_insn() at all. > We are going to replace this page, can't we save/analyze ->insn later > when we copy the content of the old page? Most probably I missed > something simple... > Copying the instruction at the time we replace the original instruction would have been ideal. However there are a few irritants to handle. - While inserting the breakpoint, we might find that the original instruction to be the breakpoint instruction itself. (This could happen if mmap_uprobe were to race with register_uprobe() or somebody else like gdb inserted a breakpoint). How do we distinguish if the breakpoint instruction was around in the text or somebody inserted a breakpoint in that address-space? Since we read from the page-cache, we can easily resolve this. - On archs like x86, with variable size instructions, the original instruction can be across 2 pages. This is because we copy the maximum instruction size from the given vaddr into a buffer for subsequent analysis. So the copy_insn takes care of getting two pages if and when required. Currently the insert and remove breakpoint assumes that the instruction size of a breakpoint is the smallest size for that architecture. Hence reading/writing to one page in write_opcode is good enough. - Again on variable instruction size supporting archs, if two subsequent instructions are probed, the original instruction if copied using get_user_pages might already have a breakpoint included. (This shouldnt have any effect on the uprobes though.) -- Thanks and Regards Srikar