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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 174AEC4338F for ; Sun, 8 Aug 2021 10:26:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E023C61019 for ; Sun, 8 Aug 2021 10:26:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230197AbhHHK1J (ORCPT ); Sun, 8 Aug 2021 06:27:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39650 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229613AbhHHK1I (ORCPT ); Sun, 8 Aug 2021 06:27:08 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B5DB6C061760 for ; Sun, 8 Aug 2021 03:26:49 -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-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=nJSEr6dgSE+SMhKjhRnVer4kjNRkJDPVR9Jv8j+ruPQ=; b=l7WuTEAsSXMuiRzv2lbDsuF6Lg aVxQWuFcSrR9Yep16udEPOdBE+BdlO43Li5RBxB8ujatBbcGhwd08/fwTGcUnmqWFrpEFhNa5aCwY 1oEkh2CZJrOnPFQQyVR7e8VjfIvy0aQGLjXe1WAhvFafyeZIR+hRFr+tOFz1CyenHD6Ish/Bx/tuy yNlVME9ArpNRBPyblHrKWVLpZTHDNe1VFz2jUHkT5JMjEU3gX2ye1n2d/A2fHWcVALnzu4zbYew94 mOhJzxaBPyR5sUal76A25754PrgRErJ4B3GlTZk1fmxnFv/TmQ+obqzFa4bWlTVTho3vGHWBjlkiY KILq2m2g==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mCg0O-009urQ-Rb; Sun, 08 Aug 2021 10:26:17 +0000 Date: Sun, 8 Aug 2021 11:26:08 +0100 From: Matthew Wilcox To: Baolin Wang Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place Message-ID: References: <1f7e1d083864fbb17a20a9c8349d2e8b427e20a3.1628174413.git.baolin.wang@linux.alibaba.com> <36956352-246a-b3c2-3ade-2a6c22e2cd5a@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <36956352-246a-b3c2-3ade-2a6c22e2cd5a@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Aug 08, 2021 at 10:55:30AM +0800, Baolin Wang wrote: > Hi, > > > On Fri, Aug 06, 2021 at 11:07:18AM +0800, Baolin Wang wrote: > > > Hi Matthew, > > > > > > > On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote: > > > > > We've got the expected count for anonymous page or file page by > > > > > expected_page_refs() at the beginning of migrate_page_move_mapping(), > > > > > thus we should move the page count validation a little forward to > > > > > reduce duplicated code. > > > > > > > > Please add an explanation to the changelog for why it's safe to pull > > > > this out from under the i_pages lock. > > > > > > Sure. In folio_migrate_mapping(), we are sure that the migration page was > > > isolated from lru list and locked, so I think there are no race to get the > > > page count without i_pages lock. Please correct me if I missed something > > > else. Thanks. > > > > Unless the page has been removed from i_pages, this isn't a correct > > explanation. Even if it has been removed from i_pages, unless an > > RCU grace period has passed, another CPU may still be able to inc the > > refcount on it (temporarily). The same is true for the page tables, > > by the way; if someone is using get_user_pages_fast(), they may still > > be able to see the page. > > I don't think this is an issue, cause now we've established a migration pte > for this migration page under page lock. If the user want to get page by > get_user_pages_fast(), it will wait for the page miggration finished by > migration_entry_wait(). So I still think there is no need to check the > migration page count under the i_pages lock. I don't know whether the patch is correct or not, but you aren't nearly paranoid enough. Consider this sequence of events: CPU 0: CPU 1: get_user_pages_fast() lockless_pages_from_mm() local_irq_save() gup_pgd_range() gup_p4d_range() gup_pud_range() gup_pmd_range() gup_pte_range() pte_t pte = ptep_get_lockless(ptep); migrate_vma_collect_pmd() ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl) ptep_get_and_clear(mm, addr, ptep); page = pte_page(pte); set_pte_at(mm, addr, ptep, swp_pte); migrate_page_move_mapping() head = try_grab_compound_head(page, 1, flags); ... now page's refcount is temporarily higher than it should be. CPU 0 will notice the PTE is no longer the PTE that it used to be and drop the reference, but in the meantime, CPU 1 can observe the higher refcount. None of this has anything to do with the i_pages lock. Holding it does not protect from this race, but you need to know this kind of thing to decide if changing how we test a page's refcount is safe or not.