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 C3AECC4338F for ; Sun, 8 Aug 2021 16:02:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9B0F561004 for ; Sun, 8 Aug 2021 16:02:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231424AbhHHQCt (ORCPT ); Sun, 8 Aug 2021 12:02:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56254 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229608AbhHHQCs (ORCPT ); Sun, 8 Aug 2021 12:02:48 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B3040C061760 for ; Sun, 8 Aug 2021 09:02:26 -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=XLXYcBeoKTT5ZkFSI8EZN5mp95T20OMCOy+Eix1Rw3k=; b=Q3EtDv9ZePEpF0v9ZRnKPrKRSi sGwwecqgRxr3YGcyu+7rnF1ZfycQpL0uAbdCkj/dhdVSaQpakWCge+7R9Fan4gvpeLxGfvUekMdvA iuI6+cPyP0Fa4/lM7jphXYaWoO+rRxoz90EzFM9gf9CwcWijIuG/neRXVibjJT8NBdxM93lKNs8xc zAquBx9crRMxdS5skLekAU8FEHPlcCS9hxkSawQ28OM821ONw/6MBq9ywN3KiMU1h0Jf6ksxtGCet IhpVcF0iSjGCH1yfYSzLTvyC/qdM0Grq1NAcmgq6MfIrxEsPaauXBF+rDPn/X8+aYDyVbUelIgKQi UsB7ReWQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mClEa-00A6ZE-CY; Sun, 08 Aug 2021 16:01:33 +0000 Date: Sun, 8 Aug 2021 17:01:08 +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> <4f25b4e9-0069-1749-32cf-d4644f13be4e@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4f25b4e9-0069-1749-32cf-d4644f13be4e@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Aug 08, 2021 at 11:13:28PM +0800, Baolin Wang wrote: > On 2021/8/8 18:26, Matthew Wilcox wrote: > > 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: > > Thanks for describing this scenario. > > > > > 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); > > On CPU0, after grab the page count, it will validate the PTE again. If swap > PTE has been established for this page, it will drop the count and go to the > slow path. > if (unlikely(pte_val(pte) != pte_val(*ptep))) { > put_compound_head(head, 1, flags); > goto pte_unmap; > } > > So CPU1 can not observe the abnormal higher refcount in this case if I did > not miss anything. This is a race between CPUs. There is no synchronisation between them, so CPU 1 can absolutely see the refcount higher temporarily. Yes, CPU 0 will eventually put the refcount, but CPU 1 can observe it high.