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 DFB90C433FE for ; Thu, 13 Oct 2022 17:57:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229965AbiJMR52 (ORCPT ); Thu, 13 Oct 2022 13:57:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54714 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230132AbiJMR4r (ORCPT ); Thu, 13 Oct 2022 13:56:47 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EC723A8CC8; Thu, 13 Oct 2022 10:55:22 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 7DCF0618DE; Thu, 13 Oct 2022 17:54:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5B381C4347C; Thu, 13 Oct 2022 17:54:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1665683675; bh=1NO7INAh9t/nqLkNlb48OrRHodPX0iieToKEAO8MS5s=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=YdJvLLLQzIe7Zs495sVDoK/q6b++D2d7Ukpc6cs+/J1CmzUBq6VGjUmytBuXfG0FK Wk75whIqbzj25LwxQp2UjPjsq/SympkSHxQP8KMYSiBnrlF3N2tDxph8pNsvuka9Fm TCaPE5ogOzGpK7WQqCoSyjFytsJ8vsRAihpytW6U= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, David Hildenbrand , Peter Xu , Yang Shi , John Hubbard , "Aneesh Kumar K.V" , Hugh Dickins , Jason Gunthorpe , "Kirill A. Shutemov" , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Andrew Morton Subject: [PATCH 5.10 11/54] mm: gup: fix the fast GUP race against THP collapse Date: Thu, 13 Oct 2022 19:52:05 +0200 Message-Id: <20221013175147.639795471@linuxfoundation.org> X-Mailer: git-send-email 2.38.0 In-Reply-To: <20221013175147.337501757@linuxfoundation.org> References: <20221013175147.337501757@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Yang Shi commit 70cbc3cc78a997d8247b50389d37c4e1736019da upstream. Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer sufficient to handle concurrent GUP-fast in all cases, it only handles traditional IPI-based GUP-fast correctly. On architectures that send an IPI broadcast on TLB flush, it works as expected. But on the architectures that do not use IPI to broadcast TLB flush, it may have the below race: CPU A CPU B THP collapse fast GUP gup_pmd_range() <-- see valid pmd gup_pte_range() <-- work on pte pmdp_collapse_flush() <-- clear pmd and flush __collapse_huge_page_isolate() check page pinned <-- before GUP bump refcount pin the page check PTE <-- no change __collapse_huge_page_copy() copy data to huge page ptep_clear() install huge pmd for the huge page return the stale page discard the stale page The race can be fixed by checking whether PMD is changed or not after taking the page pin in fast GUP, just like what it does for PTE. If the PMD is changed it means there may be parallel THP collapse, so GUP should back off. Also update the stale comment about serializing against fast GUP in khugepaged. Link: https://lkml.kernel.org/r/20220907180144.555485-1-shy828301@gmail.com Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()") Acked-by: David Hildenbrand Acked-by: Peter Xu Signed-off-by: Yang Shi Reviewed-by: John Hubbard Cc: "Aneesh Kumar K.V" Cc: Hugh Dickins Cc: Jason Gunthorpe Cc: "Kirill A. Shutemov" Cc: Michael Ellerman Cc: Nicholas Piggin Cc: Christophe Leroy Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- mm/gup.c | 34 ++++++++++++++++++++++++++++------ mm/khugepaged.c | 10 ++++++---- 2 files changed, 34 insertions(+), 10 deletions(-) --- a/mm/gup.c +++ b/mm/gup.c @@ -2128,8 +2128,28 @@ static void __maybe_unused undo_dev_page } #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL -static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, - unsigned int flags, struct page **pages, int *nr) +/* + * Fast-gup relies on pte change detection to avoid concurrent pgtable + * operations. + * + * To pin the page, fast-gup needs to do below in order: + * (1) pin the page (by prefetching pte), then (2) check pte not changed. + * + * For the rest of pgtable operations where pgtable updates can be racy + * with fast-gup, we need to do (1) clear pte, then (2) check whether page + * is pinned. + * + * Above will work for all pte-level operations, including THP split. + * + * For THP collapse, it's a bit more complicated because fast-gup may be + * walking a pgtable page that is being freed (pte is still valid but pmd + * can be cleared already). To avoid race in such condition, we need to + * also check pmd here to make sure pmd doesn't change (corresponds to + * pmdp_collapse_flush() in the THP collapse code path). + */ +static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, + unsigned long end, unsigned int flags, + struct page **pages, int *nr) { struct dev_pagemap *pgmap = NULL; int nr_start = *nr, ret = 0; @@ -2169,7 +2189,8 @@ static int gup_pte_range(pmd_t pmd, unsi if (!head) goto pte_unmap; - if (unlikely(pte_val(pte) != pte_val(*ptep))) { + if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) || + unlikely(pte_val(pte) != pte_val(*ptep))) { put_compound_head(head, 1, flags); goto pte_unmap; } @@ -2214,8 +2235,9 @@ pte_unmap: * get_user_pages_fast_only implementation that can pin pages. Thus it's still * useful to have gup_huge_pmd even if we can't operate on ptes. */ -static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, - unsigned int flags, struct page **pages, int *nr) +static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, + unsigned long end, unsigned int flags, + struct page **pages, int *nr) { return 0; } @@ -2522,7 +2544,7 @@ static int gup_pmd_range(pud_t *pudp, pu if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr, PMD_SHIFT, next, flags, pages, nr)) return 0; - } else if (!gup_pte_range(pmd, addr, next, flags, pages, nr)) + } else if (!gup_pte_range(pmd, pmdp, addr, next, flags, pages, nr)) return 0; } while (pmdp++, addr = next, addr != end); --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1144,10 +1144,12 @@ static void collapse_huge_page(struct mm pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */ /* - * After this gup_fast can't run anymore. This also removes - * any huge TLB entry from the CPU so we won't allow - * huge and small TLB entries for the same virtual address - * to avoid the risk of CPU bugs in that area. + * This removes any huge TLB entry from the CPU so we won't allow + * huge and small TLB entries for the same virtual address to + * avoid the risk of CPU bugs in that area. + * + * Parallel fast GUP is fine since fast GUP will back off when + * it detects PMD is changed. */ _pmd = pmdp_collapse_flush(vma, address, pmd); spin_unlock(pmd_ptl);