From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga05-in.huawei.com (szxga05-in.huawei.com [45.249.212.191]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 819ED5E071; Wed, 6 Mar 2024 08:51:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.191 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709715089; cv=none; b=g0+akGLxSokXPdjhkXasShYZy/E/usRKmawT2UgRovF05gwpRkFUfinnCN0HWPbrwbhBFHvDeH98tJ+sIB3Svzi+28slxEtjXRqhUFinaGLZ5fwrRVmiYFyzx4cFz+P2yGNCvSIy6LOFyE9SKCem19CtPPRYotXXEzlYL1D33zU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709715089; c=relaxed/simple; bh=xp3myRu7i4qmBkpzpPGi8j9kn865KP8v+wf/ERPHx6c=; h=Subject:To:CC:References:From:Message-ID:Date:MIME-Version: In-Reply-To:Content-Type; b=CbIiphmo014Y72Tw+v8PUPsmMJF3oOJgUC046fgq3Krxk9plaLsvCPzCJeTsgUgxAtVVPms1x0GjZgz2dxiGBo/evnYpQqqc6LOhSMElzzmD4+dGO/RdFNF93g8+wPknZaKysn7mwi3zRNYGUJ8uJmDklr19nv8wuJZpXuY14LA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=45.249.212.191 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.19.163.17]) by szxga05-in.huawei.com (SkyGuard) with ESMTP id 4TqR435cdyz1FLfD; Wed, 6 Mar 2024 16:51:15 +0800 (CST) Received: from canpemm500002.china.huawei.com (unknown [7.192.104.244]) by mail.maildlp.com (Postfix) with ESMTPS id CABD31A0172; Wed, 6 Mar 2024 16:51:23 +0800 (CST) Received: from [10.173.135.154] (10.173.135.154) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.1.2507.35; Wed, 6 Mar 2024 16:51:23 +0800 Subject: Re: [PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff() To: "Huang, Ying" , Ryan Roberts CC: Andrew Morton , David Hildenbrand , , , References: <20240305151349.3781428-1-ryan.roberts@arm.com> <875xy0842q.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Miaohe Lin Message-ID: Date: Wed, 6 Mar 2024 16:51:22 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <875xy0842q.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="windows-1252" Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To canpemm500002.china.huawei.com (7.192.104.244) On 2024/3/6 10:52, Huang, Ying wrote: > Ryan Roberts writes: > >> There was previously a theoretical window where swapoff() could run and >> teardown a swap_info_struct while a call to free_swap_and_cache() was >> running in another thread. This could cause, amongst other bad >> possibilities, swap_page_trans_huge_swapped() (called by >> free_swap_and_cache()) to access the freed memory for swap_map. >> >> This is a theoretical problem and I haven't been able to provoke it from >> a test case. But there has been agreement based on code review that this >> is possible (see link below). >> >> Fix it by using get_swap_device()/put_swap_device(), which will stall >> swapoff(). There was an extra check in _swap_info_get() to confirm that >> the swap entry was valid. This wasn't present in get_swap_device() so >> I've added it. I couldn't find any existing get_swap_device() call sites >> where this extra check would cause any false alarms. >> >> Details of how to provoke one possible issue (thanks to David Hilenbrand >> for deriving this): >> >> --8<----- >> >> __swap_entry_free() might be the last user and result in >> "count == SWAP_HAS_CACHE". >> >> swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0. >> >> So the question is: could someone reclaim the folio and turn >> si->inuse_pages==0, before we completed swap_page_trans_huge_swapped(). >> >> Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are >> still references by swap entries. >> >> Process 1 still references subpage 0 via swap entry. >> Process 2 still references subpage 1 via swap entry. >> >> Process 1 quits. Calls free_swap_and_cache(). >> -> count == SWAP_HAS_CACHE >> [then, preempted in the hypervisor etc.] >> >> Process 2 quits. Calls free_swap_and_cache(). >> -> count == SWAP_HAS_CACHE >> >> Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls >> __try_to_reclaim_swap(). >> >> __try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()-> >> put_swap_folio()->free_swap_slot()->swapcache_free_entries()-> >> swap_entry_free()->swap_range_free()-> >> ... >> WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries); >> >> What stops swapoff to succeed after process 2 reclaimed the swap cache >> but before process1 finished its call to swap_page_trans_huge_swapped()? >> >> --8<----- > > I think that this can be simplified. Even for a 4K folio, this could > happen. > > CPU0 CPU1 > ---- ---- > > zap_pte_range > free_swap_and_cache > __swap_entry_free > /* swap count become 0 */ > swapoff > try_to_unuse > filemap_get_folio > folio_free_swap > /* remove swap cache */ > /* free si->swap_map[] */ > > swap_page_trans_huge_swapped <-- access freed si->swap_map !!! Sorry for jumping the discussion here. IMHO, free_swap_and_cache is called with pte lock held. So synchronize_rcu (called by swapoff) will wait zap_pte_range to release the pte lock. So this theoretical problem can't happen. Or am I miss something? CPU0 CPU1 ---- ---- zap_pte_range pte_offset_map_lock -- spin_lock is held. free_swap_and_cache __swap_entry_free /* swap count become 0 */ swapoff try_to_unuse filemap_get_folio folio_free_swap /* remove swap cache */ percpu_ref_kill(&p->users); swap_page_trans_huge_swapped pte_unmap_unlock -- spin_lock is released. synchronize_rcu(); --> Will wait pte_unmap_unlock to be called? /* free si->swap_map[] */ Thanks.