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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EF858EEAA77 for ; Thu, 14 Sep 2023 21:24:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229588AbjINVYN (ORCPT ); Thu, 14 Sep 2023 17:24:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42926 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229491AbjINVYL (ORCPT ); Thu, 14 Sep 2023 17:24:11 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D8A21CEB for ; Thu, 14 Sep 2023 14:24:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=mZz4t+gdKYS6tcBsbAuh03VOVbJbXzYNs5f6Rm45kBw=; b=AiOe4IK887hUNxiFVqKkg+HuQW i3z1RT7BL1F1YNYNpMPfqNP2vIuq7t7QIg18sw1HJrCuX8PDvXyrqhRdbPMyvery+HcEeL2D8phK8 XGEN9Vq9H8VXiImvdy55ItXT5TZJhDDJmQWnT9sW+dE/th1uVRKRFCc7TSf3blUSJR3u3BKzHaC5I dAsl6lfiCtpZirgmkV58mkC/5hoFP5HGzoCHzguzBXajRIr2eutmkdrs7AnACjFKPMSDPvtycFZZ2 pwx2HfZy/1+R/a73pgF7BqP2Y6wgf3WJXe1LP3WeYlROfO7BUhmu9K5uIJIX0AcjdFy9SSF1hNDBZ Tr7HyGFQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qgtog-005HEe-SZ; Thu, 14 Sep 2023 21:24:02 +0000 Date: Thu, 14 Sep 2023 22:24:02 +0100 From: Matthew Wilcox To: Suren Baghdasaryan Cc: syzbot , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, syzkaller-bugs@googlegroups.com Subject: Re: [syzbot] [mm?] kernel BUG in vma_replace_policy Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 14, 2023 at 08:53:59PM +0000, Suren Baghdasaryan wrote: > On Thu, Sep 14, 2023 at 8:00 PM Suren Baghdasaryan wrote: > > > > On Thu, Sep 14, 2023 at 7:09 PM Matthew Wilcox wrote: > > > > > > On Thu, Sep 14, 2023 at 06:20:56PM +0000, Suren Baghdasaryan wrote: > > > > I think I found the problem and the explanation is much simpler. While > > > > walking the page range, queue_folios_pte_range() encounters an > > > > unmovable page and queue_folios_pte_range() returns 1. That causes a > > > > break from the loop inside walk_page_range() and no more VMAs get > > > > locked. After that the loop calling mbind_range() walks over all VMAs, > > > > even the ones which were skipped by queue_folios_pte_range() and that > > > > causes this BUG assertion. > > > > > > > > Thinking what's the right way to handle this situation (what's the > > > > expected behavior here)... > > > > I think the safest way would be to modify walk_page_range() and make > > > > it continue calling process_vma_walk_lock() for all VMAs in the range > > > > even when __walk_page_range() returns a positive err. Any objection or > > > > alternative suggestions? > > > > > > So we only return 1 here if MPOL_MF_MOVE* & MPOL_MF_STRICT were > > > specified. That means we're going to return an error, no matter what, > > > and there's no point in calling mbind_range(). Right? > > > > > > +++ b/mm/mempolicy.c > > > @@ -1334,6 +1334,8 @@ static long do_mbind(unsigned long start, unsigned long len, > > > ret = queue_pages_range(mm, start, end, nmask, > > > flags | MPOL_MF_INVERT, &pagelist, true); > > > > > > + if (ret == 1) > > > + ret = -EIO; > > > if (ret < 0) { > > > err = ret; > > > goto up_out; > > > > > > (I don't really understand this code, so it can't be this simple, can > > > it? Why don't we just return -EIO from queue_folios_pte_range() if > > > this is the right answer?) > > > > Yeah, I'm trying to understand the expected behavior of this function > > to make sure we are not missing anything. I tried a simple fix that I > > suggested in my previous email and it works but I want to understand a > > bit more about this function's logic before posting the fix. > > So, current functionality is that after queue_pages_range() encounters > an unmovable page, terminates the loop and returns 1, mbind_range() > will still be called for the whole range > (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1345), > all pages in the pagelist will be migrated > (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1355) > and only after that the -EIO code will be returned > (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1362). > So, if we follow Matthew's suggestion we will be altering the current > behavior which I assume is not what we want to do. Right, I'm intentionally changing the behaviour. My thinking is that mbind(MPOL_MF_MOVE | MPOL_MF_STRICT) is going to fail. Should such a failure actually move the movable pages before reporting that it failed? I don't know. > The simple fix I was thinking about that would not alter this behavior > is smth like this: I don't like it, but can we run it past syzbot to be sure it solves the issue and we're not chasing a ghost here?