From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_NEOMUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 41EA3C282CE for ; Wed, 22 May 2019 19:43:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 154CF2081C for ; Wed, 22 May 2019 19:43:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732887AbfEVTnn (ORCPT ); Wed, 22 May 2019 15:43:43 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:37834 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729616AbfEVTnc (ORCPT ); Wed, 22 May 2019 15:43:32 -0400 Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1hTX93-00050n-IX; Wed, 22 May 2019 21:43:25 +0200 Date: Wed, 22 May 2019 21:43:24 +0200 From: Sebastian Andrzej Siewior To: Andrew Morton Cc: Mike Rapoport , Andrea Arcangeli , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Borislav Petkov Subject: Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults Message-ID: <20190522194322.5k52docwgp5zkdcj@linutronix.de> References: <1557844195-18882-1-git-send-email-rppt@linux.ibm.com> <20190522122113.a2edc8aba32f0fad189bae21@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190522122113.a2edc8aba32f0fad189bae21@linux-foundation.org> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019-05-22 12:21:13 [-0700], Andrew Morton wrote: > On Tue, 14 May 2019 17:29:55 +0300 Mike Rapoport wrote: > > > When get_user_pages*() is called with pages = NULL, the processing of > > VM_FAULT_RETRY terminates early without actually retrying to fault-in all > > the pages. > > > > If the pages in the requested range belong to a VMA that has userfaultfd > > registered, handle_userfault() returns VM_FAULT_RETRY *after* user space > > has populated the page, but for the gup pre-fault case there's no actual > > retry and the caller will get no pages although they are present. > > > > This issue was uncovered when running post-copy memory restore in CRIU > > after commit d9c9ce34ed5c ("x86/fpu: Fault-in user stack if > > copy_fpstate_to_sigframe() fails"). > > > > After this change, the copying of FPU state to the sigframe switched from > > copy_to_user() variants which caused a real page fault to get_user_pages() > > with pages parameter set to NULL. > > You're saying that argument buf_fx in copy_fpstate_to_sigframe() is NULL? buf_fx is user stack pointer and it should not be NULL. > If so was that expected by the (now cc'ed) developers of > d9c9ce34ed5c8923 ("x86/fpu: Fault-in user stack if > copy_fpstate_to_sigframe() fails")? > > It seems rather odd. copy_fpregs_to_sigframe() doesn't look like it's > expecting a NULL argument. exactly, this is not expected. > Also, I wonder if copy_fpstate_to_sigframe() would be better using > fault_in_pages_writeable() rather than get_user_pages_unlocked(). That > seems like it operates at a more suitable level and I guess it will fix > this issue also. It looks, like fault_in_pages_writeable() would work. If this is the recommendation from the MM department than I can switch to that. > > In post-copy mode of CRIU, the destination memory is managed with > > userfaultfd and lack of the retry for pre-fault case in get_user_pages() > > causes a crash of the restored process. > > > > Making the pre-fault behavior of get_user_pages() the same as the "normal" > > one fixes the issue. > > Should this be backported into -stable trees? Sebastian