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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 4F23EC433DF for ; Thu, 28 May 2020 11:20:44 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 110EC206F1 for ; Thu, 28 May 2020 11:20:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="cnGYK2JC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 110EC206F1 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id A784A800B9; Thu, 28 May 2020 07:20:43 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 9DB1D80010; Thu, 28 May 2020 07:20:43 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8C9C0800B9; Thu, 28 May 2020 07:20:43 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0078.hostedemail.com [216.40.44.78]) by kanga.kvack.org (Postfix) with ESMTP id 754A880010 for ; Thu, 28 May 2020 07:20:43 -0400 (EDT) Received: from smtpin17.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 32642180AD81F for ; Thu, 28 May 2020 11:20:43 +0000 (UTC) X-FDA: 76865885166.17.rate19_7a9aebd789933 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin17.hostedemail.com (Postfix) with ESMTP id 09F6C180D0180 for ; Thu, 28 May 2020 11:20:43 +0000 (UTC) X-HE-Tag: rate19_7a9aebd789933 X-Filterd-Recvd-Size: 4886 Received: from us-smtp-delivery-1.mimecast.com (us-smtp-1.mimecast.com [207.211.31.81]) by imf23.hostedemail.com (Postfix) with ESMTP for ; Thu, 28 May 2020 11:20:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590664841; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0pDdsaPJ9DSWTCG29xVVgPA72+92+ZpKaGKhvHYc8/o=; b=cnGYK2JC5bYnTT2Pd08ImODjvO31mfBzx6DCuwCatJUslkIA68z3sN8V+SAI0uwCmF7riy oZ98Ejx2vo6hC9D5lt4lZqF3Ah6wX+67qWpQUied2otSd9O0NlcXwKUJNXT0AMsTpkVBtM qZOOCOQ7RFgWFcEOIQTt0/klvhEp9jc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-357-rz4JWXClNKyU_Z2vtGkSbw-1; Thu, 28 May 2020 07:20:37 -0400 X-MC-Unique: rz4JWXClNKyU_Z2vtGkSbw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C3F14805730; Thu, 28 May 2020 11:20:32 +0000 (UTC) Received: from redhat.com (ovpn-113-1.rdu2.redhat.com [10.10.113.1]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E409A7A8B6; Thu, 28 May 2020 11:20:26 +0000 (UTC) Date: Thu, 28 May 2020 07:20:25 -0400 From: Jerome Glisse To: Hugh Dickins Cc: linux-mm@kvack.org, Andrew Morton , Huang Ying , linux-kernel@vger.kernel.org, Steven Capper , Catalin Marinas , Rabin Vincent , linux-arm-kernel@lists.infradead.org, rmk+kernel@arm.linux.org.uk, Guo Ren , linux-mips@vger.kernel.org, Ralf Baechle , Paul Burton , James Hogan , Ley Foon Tan , nios2-dev@lists.rocketboards.org, linux-parisc@vger.kernel.org, Helge Deller , "James E.J. Bottomley" , Yoshinori Sato , Rich Felker , linux-sh@vger.kernel.org, "David S. Miller" , sparclinux@vger.kernel.org, Guan Xuetao , linux-xtensa@linux-xtensa.org, Max Filippov , Chris Zankel Subject: Re: Cache flush issue with page_mapping_file() and swap back shmem page ? Message-ID: <20200528112025.GA10175@redhat.com> References: <20200528002033.GB1992500@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Rspamd-Queue-Id: 09F6C180D0180 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, May 27, 2020 at 08:46:22PM -0700, Hugh Dickins wrote: > Hi Jerome, >=20 > On Wed, 27 May 2020, Jerome Glisse wrote: > > So any arch code which uses page_mapping_file() might get the wrong > > answer, this function will return NULL for a swap backed page which > > can be a shmem pages. But shmem pages can still be shared among > > multiple process (and possibly at different virtual addresses if > > mremap was use). > >=20 > > Attached is a patch that changes page_mapping_file() to return the > > shmem mapping for swap backed shmem page. I have not tested it (no > > way for me to test all those architecture) and i spotted this while > > working on something else. So i hope someone can take a closer look. >=20 > I'm certainly no expert on flush_dcache_page() and friends, but I'd > be very surprised if such a problem exists, yet has gone unnoticed > for so long. page_mapping_file() itself is fairly new, added when > a risk of crashing on a race with swapoff came in: but the previous > use of page_mapping() would have suffered equally if there were such > a cache flushinhg problem here. >=20 > And I'm afraid your patch won't do anything to help if there is a > problem: very soon after shmem calls add_to_swap_cache(), it calls > shmem_delete_from_page_cache(), which sets page->mapping to NULL. >=20 > But I can assure you that a shmem page (unlike an anon page) is never > put into swap cache while it is mapped into userspace, and never > mapped into userspace while it is still in swap cache: does that help? >=20 You are right i missed/forgot the part where shmem is never swapcache and mapped at the same time, thus page_mapping_file() can return NULL for those as they can no longer have alias mapping. Thank you Hugh J=E9r=F4me