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 1B78A3B2A4; Mon, 1 Apr 2024 16:40:08 +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=1711989608; cv=none; b=JxI+orZoKSkfL/fKUl4rbd/GVNPYBZuEDBhcIQTdFdaVDq58inzFrKG9SuYEp7R5nSYY0w/Ot0W5VEOfn4G02MMcn2RQWG6Uj9AWfoSf43fQuK6Zt08DJBjtGMfTI3dyjulde4gh0ywR6VP+3jMKiUAObTwzNduBrk6lUYEOiHY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711989608; c=relaxed/simple; bh=YdXAOEZHfnSO/XvN0etpNkHkzsd9LNAlQFIV7fDxfKY=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=fs+NIxzHg3yLzRODC//QF38NU2Gh4Cozn6nRXPpNgNJQmHHcSFy/vh2zh+8lKDWiErRTXxxHJaVaUjeyCtRm3/58tkRGXYegx2AFf1hOB5ws1ol1MTe3pKV7VnZh1zMTIR/Z0c6nXcNfgJFxSzyVKaHwIno+knKe+ctfDdMSBvs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=GUHznYeP; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="GUHznYeP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7E9D6C433F1; Mon, 1 Apr 2024 16:40:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1711989608; bh=YdXAOEZHfnSO/XvN0etpNkHkzsd9LNAlQFIV7fDxfKY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=GUHznYePE+LlLgw+L+pY8Xg0xtZ7HFilR159yvn/TIUtitFVfoDABdJJmeMC9ET1H oICvIyL+RERUxm/kWDm9tvxenP5C6Wqs4NFBJjXY9UvOP5/0M5YHNUxwO9SFnzPVui 50JGx8lWR8VGpWVsWfkEgYqF2eJOKBpiJSzARhwY= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Ryan Roberts , David Hildenbrand , "Huang, Ying" , Andrew Morton , Sasha Levin Subject: [PATCH 6.6 100/396] mm: swap: fix race between free_swap_and_cache() and swapoff() Date: Mon, 1 Apr 2024 17:42:29 +0200 Message-ID: <20240401152550.902916260@linuxfoundation.org> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20240401152547.867452742@linuxfoundation.org> References: <20240401152547.867452742@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Ryan Roberts [ Upstream commit 82b1c07a0af603e3c47b906c8e991dc96f01688e ] 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 Signed-off-by: Sasha Levin --- mm/swapfile.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/mm/swapfile.c b/mm/swapfile.c index 750314fff0c46..eada1351753e3 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -1226,6 +1226,11 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p, * 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 @@ -1603,13 +1608,19 @@ int free_swap_and_cache(swp_entry_t entry) 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; } -- 2.43.0