From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762129AbcLTRvI (ORCPT ); Tue, 20 Dec 2016 12:51:08 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42570 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757366AbcLTRun (ORCPT ); Tue, 20 Dec 2016 12:50:43 -0500 Date: Tue, 20 Dec 2016 18:50:05 +0100 From: Oleg Nesterov To: Marcin Nowakowski Cc: tip-bot for Marcin Nowakowski , linux-tip-commits@vger.kernel.org, linux-kernel@vger.kernel.org, peterz@infradead.org, alexander.shishkin@linux.intel.com, acme@kernel.org, acme@redhat.com, mingo@kernel.org, victor.kamensky@linaro.org, jolsa@redhat.com, hpa@zytor.com, torvalds@linux-foundation.org, tglx@linutronix.de Subject: Re: [tip:perf/urgent] uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation Message-ID: <20161220175004.GB7776@redhat.com> References: <1481625657-22850-1-git-send-email-marcin.nowakowski@imgtec.com> <20161220130820.GA24822@redhat.com> <8707b178-15e0-1ca0-5612-f42d771446cc@imgtec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8707b178-15e0-1ca0-5612-f42d771446cc@imgtec.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 20 Dec 2016 17:50:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/20, Marcin Nowakowski wrote: > > Hi Oleg, > > On 20.12.2016 14:08, Oleg Nesterov wrote: >> On 12/19, tip-bot for Marcin Nowakowski wrote: >>> >>> uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation >>> >>> Commit: >>> >>> 72e6ae285a1d ('ARM: 8043/1: uprobes need icache flush after xol write' >>> >>> ... has introduced an arch-specific method to ensure all caches are >>> flushed appropriately after an instruction is written to an XOL page. >> >> when this page is already mmaped, >> >>> However, when the XOL area is created and the out-of-line breakpoint >>> instruction is copied, caches are not flushed at all and stale data may >>> be found in icache. >> >> but in this case the page is not mmaped yet, the probed application will >> take a page fault if it tries to execute this insn, > > In case of MIPS (and AFAICT ARM as well, and these are the only > architectures that implement arch_uprobe_copy_ixol), the cache flushing > is done through the kernel addresses of that page, so the fact that it > is not mapped yet is not an issue. OK, thanks, > Do I understand correctly that your statement implies that after the > page fault and mmapping the xol page, the page is guaranteed to be > updated in the cache? As definitely that is not something that is > happening at the moment. Well, I do not know. Let me repeat I don't understand this flush_.*cache magic. But. do_read_fault() does __do_fault(..., &fault_page, ...); alloc_set_pte(fault_page); and alloc_set_pte() does flush_icache_page(vma, page)... Hmm, which is nop on MIPS. >> OK, I know nothing about MIPS, but could you help me understand this change? >> >> See above. If we really need flush_icache_range() here then perhaps we should >> modify install_special_mapping() and/or __do_fault/special_mapping_fault paths >> instead? > > Are you suggesting that those should be updated to force a cache update? Again, I do not know. But perhaps it makes more sense to actually implement flush_icache_page() ? Otherwise another user of install_special_mapping() can hit the same problem? Oleg.