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=-5.5 required=3.0 tests=MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 47751C3A59E for ; Mon, 2 Sep 2019 13:21:08 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 09FA9208CB for ; Mon, 2 Sep 2019 13:21:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 09FA9208CB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 9F44F6B0007; Mon, 2 Sep 2019 09:21:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 9A4186B0008; Mon, 2 Sep 2019 09:21:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8B9176B000A; Mon, 2 Sep 2019 09:21:07 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0099.hostedemail.com [216.40.44.99]) by kanga.kvack.org (Postfix) with ESMTP id 69E4A6B0007 for ; Mon, 2 Sep 2019 09:21:07 -0400 (EDT) Received: from smtpin24.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 8DEB23D01 for ; Mon, 2 Sep 2019 13:21:06 +0000 (UTC) X-FDA: 75890041332.24.anger92_43d84ed47a246 X-HE-Tag: anger92_43d84ed47a246 X-Filterd-Recvd-Size: 2685 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf30.hostedemail.com (Postfix) with ESMTP for ; Mon, 2 Sep 2019 13:21:06 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A855BADF1; Mon, 2 Sep 2019 13:21:04 +0000 (UTC) Date: Mon, 2 Sep 2019 15:21:04 +0200 From: Michal Hocko To: Vinayak Menon Cc: minchan@kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: fix the race between swapin_readahead and SWP_SYNCHRONOUS_IO path Message-ID: <20190902132104.GJ14028@dhcp22.suse.cz> References: <1567169011-4748-1-git-send-email-vinmenon@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1567169011-4748-1-git-send-email-vinmenon@codeaurora.org> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Fri 30-08-19 18:13:31, Vinayak Menon wrote: > The following race is observed due to which a processes faulting > on a swap entry, finds the page neither in swapcache nor swap. This > causes zram to give a zero filled page that gets mapped to the > process, resulting in a user space crash later. > > Consider parent and child processes Pa and Pb sharing the same swap > slot with swap_count 2. Swap is on zram with SWP_SYNCHRONOUS_IO set. > Virtual address 'VA' of Pa and Pb points to the shared swap entry. > > Pa Pb > > fault on VA fault on VA > do_swap_page do_swap_page > lookup_swap_cache fails lookup_swap_cache fails > Pb scheduled out > swapin_readahead (deletes zram entry) > swap_free (makes swap_count 1) > Pb scheduled in > swap_readpage (swap_count == 1) > Takes SWP_SYNCHRONOUS_IO path > zram enrty absent > zram gives a zero filled page This sounds like a zram issue, right? Why is a generic swap path changed then? > > Fix this by reading the swap_count before lookup_swap_cache, which conforms > with the order in which page is added to swap cache and swap count is > decremented in do_swap_page. In the race case above, this will let Pb take > the readahead path and thus pick the proper page from swapcache. > > Signed-off-by: Vinayak Menon -- Michal Hocko SUSE Labs