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=-9.7 required=3.0 tests=DATE_IN_FUTURE_06_12, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham 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 20EC9C4360C for ; Tue, 8 Oct 2019 16:43:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 019EE21721 for ; Tue, 8 Oct 2019 16:43:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728342AbfJHQnp (ORCPT ); Tue, 8 Oct 2019 12:43:45 -0400 Received: from ex13-edg-ou-002.vmware.com ([208.91.0.190]:43009 "EHLO EX13-EDG-OU-002.vmware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728182AbfJHQnp (ORCPT ); Tue, 8 Oct 2019 12:43:45 -0400 Received: from sc9-mailhost3.vmware.com (10.113.161.73) by EX13-EDG-OU-002.vmware.com (10.113.208.156) with Microsoft SMTP Server id 15.0.1156.6; Tue, 8 Oct 2019 09:43:42 -0700 Received: from akaher-lnx-dev.eng.vmware.com (unknown [10.110.19.203]) by sc9-mailhost3.vmware.com (Postfix) with ESMTP id 932EB40AF8; Tue, 8 Oct 2019 09:43:39 -0700 (PDT) From: Ajay Kaher To: CC: , , , , , , , , , , , , , , , , , , , Jann Horn , Subject: [PATCH v2 1/8] mm: make page ref count overflow check tighter and more explicit Date: Wed, 9 Oct 2019 06:14:16 +0530 Message-ID: <1570581863-12090-2-git-send-email-akaher@vmware.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1570581863-12090-1-git-send-email-akaher@vmware.com> References: <1570581863-12090-1-git-send-email-akaher@vmware.com> MIME-Version: 1.0 Content-Type: text/plain Received-SPF: None (EX13-EDG-OU-002.vmware.com: akaher@vmware.com does not designate permitted sender hosts) Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Linus Torvalds commit f958d7b528b1b40c44cfda5eabe2d82760d868c3 upsteam. We have a VM_BUG_ON() to check that the page reference count doesn't underflow (or get close to overflow) by checking the sign of the count. That's all fine, but we actually want to allow people to use a "get page ref unless it's already very high" helper function, and we want that one to use the sign of the page ref (without triggering this VM_BUG_ON). Change the VM_BUG_ON to only check for small underflows (or _very_ close to overflowing), and ignore overflows which have strayed into negative territory. Acked-by: Matthew Wilcox Cc: Jann Horn Cc: stable@kernel.org Signed-off-by: Linus Torvalds [ 4.4.y backport notes: Ajay: Open-coded atomic refcount access due to missing page_ref_count() helper in 4.4.y Srivatsa: Added overflow check to get_page_foll() and related code. ] Signed-off-by: Srivatsa S. Bhat (VMware) Signed-off-by: Ajay Kaher --- include/linux/mm.h | 6 +++++- mm/internal.h | 5 +++-- 2 files changed, 8 insertions(+), 3 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index ed653ba..701088e 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -488,6 +488,10 @@ static inline void get_huge_page_tail(struct page *page) extern bool __get_page_tail(struct page *page); +/* 127: arbitrary random number, small enough to assemble well */ +#define page_ref_zero_or_close_to_overflow(page) \ + ((unsigned int) atomic_read(&page->_count) + 127u <= 127u) + static inline void get_page(struct page *page) { if (unlikely(PageTail(page))) @@ -497,7 +501,7 @@ static inline void get_page(struct page *page) * Getting a normal page or the head of a compound page * requires to already have an elevated page->_count. */ - VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page); + VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page); atomic_inc(&page->_count); } diff --git a/mm/internal.h b/mm/internal.h index f63f439..67015e5 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -81,7 +81,8 @@ static inline void __get_page_tail_foll(struct page *page, * speculative page access (like in * page_cache_get_speculative()) on tail pages. */ - VM_BUG_ON_PAGE(atomic_read(&compound_head(page)->_count) <= 0, page); + VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(compound_head(page)), + page); if (get_page_head) atomic_inc(&compound_head(page)->_count); get_huge_page_tail(page); @@ -106,7 +107,7 @@ static inline void get_page_foll(struct page *page) * Getting a normal page or the head of a compound page * requires to already have an elevated page->_count. */ - VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page); + VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page); atomic_inc(&page->_count); } } -- 2.7.4