From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 A87D81862F; Wed, 6 Mar 2024 21:05:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709759127; cv=none; b=iUV2vqGB+XJ3axE6kkMW+ax4MBDfkfhpOY0Y1whdQFCDsBCzWRv17/P5I5HpxrTs5Uza6UOBYc2EqiYfWxiHmYO2pUFxc9PYIH87C0QKx9ZLy4DyEo5siMW52CcyvpLpOsMH30AwXkNJAOQJBLJfJW0iyF/JPwQIhrPVKn1AVoA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709759127; c=relaxed/simple; bh=tIjys/FUQaWw4XD+wpR/5C9NgGzIzIubyk8iZgGNAiQ=; h=Date:To:From:Subject:Message-Id; b=Z4/YqSkle19d/IF2MNst6ZJ8s9I385ORfCR6AtaVnG2lGUQeFQ2MwSCjjReh37OYSqo3TMI8vEPD1JLvpKTT/TFVX0j1m4T2+vabSCz+urM02jiBOOO9Mxxn0hThukcyiDq15uTVaISsPLTV78LRFi/nUb+axLwTleRwirmnu08= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=bj42NrcM; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="bj42NrcM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7AD86C433F1; Wed, 6 Mar 2024 21:05:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1709759127; bh=tIjys/FUQaWw4XD+wpR/5C9NgGzIzIubyk8iZgGNAiQ=; h=Date:To:From:Subject:From; b=bj42NrcM5qr41jUgvHdnSkZQVE80wvhIzAA0LUqfynLfEGi5OAxoXAmlz1o+YwTQN F0LQ7oUWrzt3sp6Pcd12ITYXHeZn+q5zik8EODuDN6pnAGFOMigMQp2Nua29Jk4WNU K+gr1qA0ycAr6u1IENFZdeRxzVzvxOmiK8WlqLSw= Date: Wed, 06 Mar 2024 13:05:26 -0800 To: mm-commits@vger.kernel.org,ying.huang@intel.com,stable@vger.kernel.org,david@redhat.com,ryan.roberts@arm.com,akpm@linux-foundation.org From: Andrew Morton Subject: [merged mm-stable] mm-swap-fix-race-between-free_swap_and_cache-and-swapoff.patch removed from -mm tree Message-Id: <20240306210527.7AD86C433F1@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The quilt patch titled Subject: mm: swap: fix race between free_swap_and_cache() and swapoff() has been removed from the -mm tree. Its filename was mm-swap-fix-race-between-free_swap_and_cache-and-swapoff.patch This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm ------------------------------------------------------ From: Ryan Roberts Subject: mm: swap: fix race between free_swap_and_cache() and swapoff() Date: Wed, 6 Mar 2024 14:03:56 +0000 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 not free. This isn't present in get_swap_device() because it doesn't make sense in general due to the race between getting the reference and swapoff. So I've added an equivalent check directly in free_swap_and_cache(). Details of how to provoke one possible issue (thanks to David Hildenbrand 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<----- Link: https://lkml.kernel.org/r/20240306140356.3974886-1-ryan.roberts@arm.com Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch") Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@redhat.com/ Signed-off-by: Ryan Roberts Cc: David Hildenbrand Cc: "Huang, Ying" Cc: Signed-off-by: Andrew Morton --- mm/swapfile.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) --- a/mm/swapfile.c~mm-swap-fix-race-between-free_swap_and_cache-and-swapoff +++ a/mm/swapfile.c @@ -1232,6 +1232,11 @@ static unsigned char __swap_entry_free_l * with get_swap_device() and put_swap_device(), unless the swap * functions call get/put_swap_device() by themselves. * + * Note that when only holding the PTL, swapoff might succeed immediately + * after freeing a swap entry. Therefore, immediately after + * __swap_entry_free(), the swap info might become stale and should not + * be touched without a prior get_swap_device(). + * * Check whether swap entry is valid in the swap device. If so, * return pointer to swap_info_struct, and keep the swap entry valid * via preventing the swap device from being swapoff, until @@ -1609,13 +1614,19 @@ int free_swap_and_cache(swp_entry_t entr if (non_swap_entry(entry)) return 1; - p = _swap_info_get(entry); + p = get_swap_device(entry); if (p) { + if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) { + put_swap_device(p); + return 0; + } + count = __swap_entry_free(p, entry); if (count == SWAP_HAS_CACHE && !swap_page_trans_huge_swapped(p, entry)) __try_to_reclaim_swap(p, swp_offset(entry), TTRS_UNMAPPED | TTRS_FULL); + put_swap_device(p); } return p != NULL; } _ Patches currently in -mm which might be from ryan.roberts@arm.com are