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=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 7F58CC4320A for ; Tue, 3 Aug 2021 08:07:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 69A3360F70 for ; Tue, 3 Aug 2021 08:07:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234438AbhHCIHO (ORCPT ); Tue, 3 Aug 2021 04:07:14 -0400 Received: from mga06.intel.com ([134.134.136.31]:3408 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234440AbhHCIHH (ORCPT ); Tue, 3 Aug 2021 04:07:07 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10064"; a="274682761" X-IronPort-AV: E=Sophos;i="5.84,291,1620716400"; d="scan'208";a="274682761" Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 Aug 2021 01:06:53 -0700 X-IronPort-AV: E=Sophos;i="5.84,291,1620716400"; d="scan'208";a="510938473" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.239.159.119]) by fmsmga003-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 Aug 2021 01:06:49 -0700 From: "Huang, Ying" To: Andrew Morton Cc: Hugh Dickins , David Hildenbrand , Yang Shi , Linux-MM , LKML , Miaohe Lin , Johannes Weiner , Michal Hocko , Joonsoo Kim , Matthew Wilcox , Minchan Kim , huang ying Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page() References: <20210723080000.93953-1-ying.huang@intel.com> <24187e5e-069-9f3f-cefe-39ac70783753@google.com> Date: Tue, 03 Aug 2021 16:06:47 +0800 In-Reply-To: (huang ying's message of "Wed, 28 Jul 2021 21:03:50 +0800") Message-ID: <877dh354vc.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Andrew, huang ying writes: >> >> Thanks for catching that; and as David says, it's worse than a typo. >> >> But this is not the right fix: >> 2efa33fc7f6e ("mm/shmem: fix shmem_swapin() race with swapoff") >> needs to be reverted. >> >> It's been on my pile to look at for weeks: now I look at it and see >> it's just a bad patch. Over-enthusiastic stablehands already rushed >> it out, I was wary, and reverts are already in -rc for 5.13 and 5.10, >> phew, but 5.12.19 EOL is stuck with it unfortunately, oh well. >> >> I was wary because, if the (never observed) race to be fixed is in >> swap_cluster_readahead(), why was shmem_swapin_page() being patched? > > When we get a swap entry from the page table or shmem xarray, and no > necessary lock is held to prevent the swap device to be swapoff (e.g. > page table lock, page lock, etc.), it's possible that the swap device > has been swapoff when we operate on the swap entry (e.g. swapin). So > we need to find a way to prevent the swap device to be swapoff, > get_swap_device() based on percpu_ref is used for that. To avoid to > call get_swap_device() here and there (e.g. now it is called in many > different places), I think it's better to call get_swap_device() when > we just get a swap entry without holding the necessary lock, that is, > in do_swap_page() and shmem_swapin_page(), etc. So that we can delete > the get_swap_device() call in lookup_swap_cache(), > __read_swap_cache_async(), etc. This will make it easier to > understand when to use get_swap_device() and clean up the code. Do > you agree? > >> Not explained in its commit message, probably a misunderstanding of >> how mm/shmem.c already manages races (and prefers not to be involved >> in swap_info_struct stuff). > > Yes. The commit message isn't clean enough about why we do that. > >> But why do I now say it's bad? Because even if you correct the EINVAL >> to -EINVAL, that's an unexpected error: -EEXIST is common, -ENOMEM is >> not surprising, -ENOSPC can need consideration, but -EIO and anything >> else just end up as SIGBUS when faulting (or as error from syscall). > > Yes. -EINVAL isn't a good choice. If it's the swapoff race, then > retrying can fix the race, so -EAGAIN may be a choice. But if the > swap entry is really invalid (almost impossible in theory), we may > need something else, for example, WARN_ON_ONCE() and SIGBUS? This > reminds me that we may need to distinguish the two possibilities in > get_swap_device()? As Hugh pointed out, EINVAL isn't an appropriate error code for race condition. After checking the code, I found that EEXIST is the error code used for race condition. So I revise the patch as below. If Hugh doesn't object, can you help to replace the patch with the below one? Best Regards, Huang, Ying -----------------------------8<--------------------------------------- >From e2b281a0b09d34d6463942e214e577ed9357c213 Mon Sep 17 00:00:00 2001 From: Huang Ying Date: Tue, 3 Aug 2021 10:51:16 +0800 Subject: [PATCH] shmem_swapin_page(): fix error processing for get_swap_device() Firstly, "-" is missing before the error code. Secondly, EINVAL isn't the proper error code for the race condition. EEXIST is used in shmem_swapin_page() for that. So the error code is changed to EEXIST too. Link: https://lkml.kernel.org/r/20210723080000.93953-1-ying.huang@intel.com Fixes: 2efa33fc7f6e ("mm/shmem: fix shmem_swapin() race with swapoff") Signed-off-by: "Huang, Ying" Cc: Miaohe Lin Cc: Hugh Dickins Cc: Johannes Weiner Cc: Michal Hocko Cc: Joonsoo Kim Cc: Matthew Wilcox Cc: Minchan Kim Cc: David Hildenbrand --- mm/shmem.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/shmem.c b/mm/shmem.c index dcc07d14162e..ba925baa4404 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1711,8 +1711,8 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index, /* Prevent swapoff from happening to us. */ si = get_swap_device(swap); if (!si) { - error = EINVAL; - goto failed; + error = -EEXIST; + goto unlock; } /* Look it up and read it in.. */ page = lookup_swap_cache(swap, NULL, 0); -- 2.30.2