From patchwork Fri Jan 31 14:06:07 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Filipe Manana X-Patchwork-Id: 11359975 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 6EA0B924 for ; Fri, 31 Jan 2020 14:06:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 430F420705 for ; Fri, 31 Jan 2020 14:06:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580479573; bh=GgEqBqZxE2MNwhuKD28Eqf66N4/N+s+0oIGlcICJvYY=; h=From:To:Subject:Date:List-ID:From; b=r2FMAYYZiaw9Wb+tPpd0/B6qk5TXQEk7VIaBlqoyBu817O8F572g3yaCitHYzqVSo kFFiGJXzzLBwK4jgk03jb8Jz7rynNbdSPmBPpPRb+8Xx4EaiTg6I+3wAFa8Z3nTLgm MSYqQmVagpPfzwyKOdnyjnlklRqlZsJ4ytLnhiqs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728752AbgAaOGM (ORCPT ); Fri, 31 Jan 2020 09:06:12 -0500 Received: from mail.kernel.org ([198.145.29.99]:43476 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728500AbgAaOGM (ORCPT ); Fri, 31 Jan 2020 09:06:12 -0500 Received: from debian6.Home (bl8-197-74.dsl.telepac.pt [85.241.197.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 51A1D2067C for ; Fri, 31 Jan 2020 14:06:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580479570; bh=GgEqBqZxE2MNwhuKD28Eqf66N4/N+s+0oIGlcICJvYY=; h=From:To:Subject:Date:From; b=srCfNF9o89wVZDIRM7LN3B++RK+ZvK558XyQqKqAk+iCdPaVoVPqkMSErSngmlVH1 mqbUCJY2DDyQNcHKRfVyFWsShZ++XfOesFfM5guX+o4u3fHI6ad17M2s0InthfHmGj Mpf3wQMRGZ3aS4aah5gNO90rpxVMDzHB8+yIfoeU= From: fdmanana@kernel.org To: linux-btrfs@vger.kernel.org Subject: [PATCH] Btrfs: fix race between using extent maps and merging them Date: Fri, 31 Jan 2020 14:06:07 +0000 Message-Id: <20200131140607.26923-1-fdmanana@kernel.org> X-Mailer: git-send-email 2.11.0 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org From: Filipe Manana We have a few cases where we allow an extent map that is in an extent map tree to be merged with other extents in the tree. Such cases include the unpinning of an extent after the respective ordered extent completed or after logging an extent during a fast fsync. This can lead to subtle and dangerous problems because when doing the merge some other task might be using the same extent map and as consequence see an inconsistent state of the extent map - for example sees the new length but has seen the old start offset. With luck this triggers a BUG_ON(), and not some silent bug, such as the following one in __do_readpage(): $ cat -n fs/btrfs/extent_io.c 3061 static int __do_readpage(struct extent_io_tree *tree, 3062 struct page *page, (...) 3127 em = __get_extent_map(inode, page, pg_offset, cur, 3128 end - cur + 1, get_extent, em_cached); 3129 if (IS_ERR_OR_NULL(em)) { 3130 SetPageError(page); 3131 unlock_extent(tree, cur, end); 3132 break; 3133 } 3134 extent_offset = cur - em->start; 3135 BUG_ON(extent_map_end(em) <= cur); (...) Consider the following example scenario, where we end up hitting the BUG_ON() in __do_readpage(). We have an inode with a size of 8Kb and 2 extent maps: extent A: file offset 0, length 4Kb, disk_bytenr = X, persisted on disk by a previous transaction extent B: file offset 4Kb, length 4Kb, disk_bytenr = X + 4Kb, not yet persisted but writeback started for it already. The extent map is pinned since there's writeback and an ordered extent in progress, so it can not be merged with extent map A yet The following sequence of steps leads to the BUG_ON(): 1) The ordered extent for extent B completes, the respective page gets its writeback bit cleared and the extent map is unpinned, at that point it is not yet merged with extent map A because it's in the list of modified extents; 2) Due to memory pressure, or some other reason, the mm subsystem releases the page corresponding to extent B - btrfs_releasepage() is called and returns 1, meaning the page can be released as it's not dirty, not under writeback anymore and the extent range is not locked in the inode's iotree. However the extent map is not released, either because we are not in a context that allows memory allocations to block or because the inode's size is smaller than 16Mb - in this case our inode has a size of 8Kb; 3) Task B needs to read extent B and ends up __do_readpage() through the btrfs_readpage() callback. At __do_readpage() it gets a reference to extent map B; 4) Task A, doing a fast fsync, calls clear_em_loggin() against extent map B while holding the write lock on the inode's extent map tree - this results in try_merge_map() being called and since it's possible to merge extent map B with extent map A now (the extent map B was removed from the list of modified extents), the merging begins - it sets extent map B's start offset to 0 (was 4Kb), but before it increments the map's length to 8Kb (4kb + 4Kb), task A is at: BUG_ON(extent_map_end(em) <= cur); The call to extent_map_end() sees the extent map has a start of 0 and a length still at 4Kb, so it returns 4Kb and 'cur' is 4Kb, so the BUG_ON() is triggered. So it's dangerous to modify an extent map that is in the tree, because some other task might have got a reference to it before and still using it, and needs to see a consistent map while using it. Generally this is very rare since most paths that lookup and use extent maps also have the file range locked in the inode's iotree. The fsync path is pretty much the only exception where we don't do it to avoid serialization with concurrent reads. Fix this by not allowing an extent map do be merged if if it's being used by tasks other then the one attempting to merge the extent map (when the reference count of the extent map is greater than 2). Reported-by: ryusuke1925 Reported-by: Koki Mitani Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=206211 CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Filipe Manana Reviewed-by: Josef Bacik --- fs/btrfs/extent_map.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c index 6f417ff68980..bd6229fb2b6f 100644 --- a/fs/btrfs/extent_map.c +++ b/fs/btrfs/extent_map.c @@ -237,6 +237,17 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em) struct extent_map *merge = NULL; struct rb_node *rb; + /* + * We can't modify an extent map that is in the tree and that is being + * used by another task, as it can cause that other task to see it in + * inconsistent state during the merging. We always have 1 reference for + * the tree and 1 for this task (which is unpinning the extent map or + * clearing the logging flag), so anything > 2 means it's being used by + * other tasks too. + */ + if (refcount_read(&em->refs) > 2) + return; + if (em->start != 0) { rb = rb_prev(&em->rb_node); if (rb)