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Tue, 17 Sep 2019 05:38:54 +0000 (UTC) Received: by smtp.codeaurora.org (Postfix, from userid 1000) id 5F16E61214; Tue, 17 Sep 2019 05:38:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=codeaurora.org; s=default; t=1568698733; bh=G4M6bBItYiShytP/ELHO/FnLmji9Zmn/YWjMGe5ISW4=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=jNss6vdrj0Jq1/3SMQUwj3i9EasDBJ3VqMnPcjQFAxAOXb3Og94ZwC/OGx5zR+b/L umJSwgyUsLGob57gNqJg4w1YVEfeVvIgIVHzmB7yKh386jU4AOQRGo5ezIVsUSqJTC fzpswlMtXJR2VdHX8AB/KteDq85vLNaqij5wK/fk= Received: from [10.204.83.131] (blr-c-bdr-fw-01_globalnat_allzones-outside.qualcomm.com [103.229.19.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: vinmenon@smtp.codeaurora.org) by smtp.codeaurora.org (Postfix) with ESMTPSA id 3243A611FD; Tue, 17 Sep 2019 05:38:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=codeaurora.org; s=default; t=1568698733; bh=G4M6bBItYiShytP/ELHO/FnLmji9Zmn/YWjMGe5ISW4=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=jNss6vdrj0Jq1/3SMQUwj3i9EasDBJ3VqMnPcjQFAxAOXb3Og94ZwC/OGx5zR+b/L umJSwgyUsLGob57gNqJg4w1YVEfeVvIgIVHzmB7yKh386jU4AOQRGo5ezIVsUSqJTC fzpswlMtXJR2VdHX8AB/KteDq85vLNaqij5wK/fk= DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 3243A611FD Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=vinmenon@codeaurora.org Subject: Re: [PATCH] mm: fix the race between swapin_readahead and SWP_SYNCHRONOUS_IO path To: Minchan Kim Cc: linux-mm@kvack.org References: <1567169011-4748-1-git-send-email-vinmenon@codeaurora.org> <20190909232613.GA39783@google.com> <9df3bb51-2094-c849-8171-dce6784e1e70@codeaurora.org> <20190910175116.GB39783@google.com> <20190912171400.GA119788@google.com> <3a500b81-71bb-54bd-9f2f-ab89ee723879@codeaurora.org> <20190916200555.GA254094@google.com> From: Vinayak Menon Message-ID: <4788d556-1b53-8d3e-121c-de2c286bac43@codeaurora.org> Date: Tue, 17 Sep 2019 11:08:49 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20190916200555.GA254094@google.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US 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 9/17/2019 1:35 AM, Minchan Kim wrote: > Hi Vinayak, > > On Fri, Sep 13, 2019 at 02:35:41PM +0530, Vinayak Menon wrote: >> On 9/12/2019 10:44 PM, Minchan Kim wrote: >>> Hi Vinayak, >>> >>> On Wed, Sep 11, 2019 at 03:37:23PM +0530, Vinayak Menon wrote: >>> >>> < snip > >>> >>>>>> Can swapcache check be done like below, before taking the SWP_SYNCHRONOUS_IO path, as an alternative ? >>>>> With your approach, what prevent below scenario? >>>>> >>>>> A B >>>>> >>>>> do_swap_page >>>>> SWP_SYNCHRONOUS_IO && __swap_count == 1 >>>> As shrink_page_list is picking the page from LRU and B is trying to read from swap simultaneously, I assume someone had read >>>> >>>> the page from swap prior to B, when its swap_count was say 2 (for it to be reclaimed by shrink_page_list now) >>> It could happen after B saw __swap_count == 1. Think about forking new process. >>> In that case, swap_count is 2 and the forked process will access the page(it >>> ends up freeing zram slot but the page would be swap cache. However, B process >>> doesn't know it). >> >> Okay, so when B has read __swap_count == 1, it means that it has taken down_read on mmap_sem in fault path >> >> already. This means fork will not be able to proceed which needs to have down_write on parent's mmap_sem ? >> > You are exactly right. However, I still believe better option to solve > the issue is to check swap_count and delte only if swap_count == 1 > in swap_slot_free_notify because it's zram specific issue and more safe > without depending other lock scheme. Sure. Let me know if you want me to post a patch for that.