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=-9.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_PASS,USER_AGENT_GIT autolearn=ham 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 9A13AC10F11 for ; Mon, 15 Apr 2019 08:29:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 663DB20651 for ; Mon, 15 Apr 2019 08:29:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1555316947; bh=aT2S75WC6rXiU9V6vQBxBka+egbS1F+jaDDzpsahr8s=; h=From:To:Subject:Date:List-ID:From; b=R0+8ef9M0V4xYbbDah2xE9KhxakPfehIlh6GZYrb3duHYlTqoGhwJQUaGXX3g5FWb wgLVqJn9soywGnWWbMr8o43onPJPMCP7AJ3mIgrDq6KiA7xIasRMDQihfveXIq3bry wb14CjoR7Bplsiel07lo44sPIbyffnI77n5dx2VE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726839AbfDOI3G (ORCPT ); Mon, 15 Apr 2019 04:29:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:55402 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726811AbfDOI3G (ORCPT ); Mon, 15 Apr 2019 04:29:06 -0400 Received: from localhost.localdomain (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 282AC20651 for ; Mon, 15 Apr 2019 08:29:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1555316944; bh=aT2S75WC6rXiU9V6vQBxBka+egbS1F+jaDDzpsahr8s=; h=From:To:Subject:Date:From; b=b5ues33ncPio7OuvH8eZnx3so0gRgvCkxQmSYQy8wRO6xX2QocBepOQLK9//pG+XV Yudk8eCWRcHWcb5Tr8JcUXIiuboSZOffuiPMPNFZH2syDAtpJE1B+Zq7jLgUJ47s9Z OkeVTrVr7G9vvdNGylipgBkZJXZ1VlE2b+kOIR2g= From: fdmanana@kernel.org To: linux-btrfs@vger.kernel.org Subject: [PATCH] Btrfs: do not start a transaction during fiemap Date: Mon, 15 Apr 2019 09:29:00 +0100 Message-Id: <20190415082900.2023-1-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 During fiemap, for regular extents (non inline) we need to check if they are shared and if they are, set the shared bit. Checking if an extent is shared requires checking the delayed references of the currently running transaction, since some reference might have not yet hit the extent tree and be only in the in-memory delayed references. However we were using a transaction join for this, which creates a new transaction when there is no transaction currently running. That means that two more potential failures can happen: creating the transaction and committing it. Further, if no write activity is currently happening in the system, and fiemap calls keep being done, we end up creating and committing transactions that do nothing. In some extreme cases this can result in the commit of the transaction created by fiemap to fail with ENOSPC when updating the root item of a subvolume tree because a join does not reserve any space, leading to a trace like the following: heisenberg kernel: ------------[ cut here ]------------ heisenberg kernel: BTRFS: Transaction aborted (error -28) heisenberg kernel: WARNING: CPU: 0 PID: 7137 at fs/btrfs/root-tree.c:136 btrfs_update_root+0x22b/0x320 [btrfs] (...) heisenberg kernel: CPU: 0 PID: 7137 Comm: btrfs-transacti Not tainted 4.19.0-4-amd64 #1 Debian 4.19.28-2 heisenberg kernel: Hardware name: FUJITSU LIFEBOOK U757/FJNB2A5, BIOS Version 1.21 03/19/2018 heisenberg kernel: RIP: 0010:btrfs_update_root+0x22b/0x320 [btrfs] (...) heisenberg kernel: RSP: 0018:ffffb5448828bd40 EFLAGS: 00010286 heisenberg kernel: RAX: 0000000000000000 RBX: ffff8ed56bccef50 RCX: 0000000000000006 heisenberg kernel: RDX: 0000000000000007 RSI: 0000000000000092 RDI: ffff8ed6bda166a0 heisenberg kernel: RBP: 00000000ffffffe4 R08: 00000000000003df R09: 0000000000000007 heisenberg kernel: R10: 0000000000000000 R11: 0000000000000001 R12: ffff8ed63396a078 heisenberg kernel: R13: ffff8ed092d7c800 R14: ffff8ed64f5db028 R15: ffff8ed6bd03d068 heisenberg kernel: FS: 0000000000000000(0000) GS:ffff8ed6bda00000(0000) knlGS:0000000000000000 heisenberg kernel: CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 heisenberg kernel: CR2: 00007f46f75f8000 CR3: 0000000310a0a002 CR4: 00000000003606f0 heisenberg kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 heisenberg kernel: DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 heisenberg kernel: Call Trace: heisenberg kernel: commit_fs_roots+0x166/0x1d0 [btrfs] heisenberg kernel: ? _cond_resched+0x15/0x30 heisenberg kernel: ? btrfs_run_delayed_refs+0xac/0x180 [btrfs] heisenberg kernel: btrfs_commit_transaction+0x2bd/0x870 [btrfs] heisenberg kernel: ? start_transaction+0x9d/0x3f0 [btrfs] heisenberg kernel: transaction_kthread+0x147/0x180 [btrfs] heisenberg kernel: ? btrfs_cleanup_transaction+0x530/0x530 [btrfs] heisenberg kernel: kthread+0x112/0x130 heisenberg kernel: ? kthread_bind+0x30/0x30 heisenberg kernel: ret_from_fork+0x35/0x40 heisenberg kernel: ---[ end trace 05de912e30e012d9 ]--- Since fiemap (and btrfs_check_shared()) is a read-only operation, do not do a transaction join to avoid the overhead of creating a new transaction (if there is currently no running transaction) and introducing a potential point of failure when the new transaction gets committed, instead use a transaction attach to grab a handle for the currently running transaction if any. Reported-by: Christoph Anton Mitterer Link: https://lore.kernel.org/linux-btrfs/b2a668d7124f1d3e410367f587926f622b3f03a4.camel@scientia.net/ Fixes: afce772e87c36c ("btrfs: fix check_shared for fiemap ioctl") Signed-off-by: Filipe Manana --- fs/btrfs/backref.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c index 11459fe84a29..876e6bb93797 100644 --- a/fs/btrfs/backref.c +++ b/fs/btrfs/backref.c @@ -1460,8 +1460,8 @@ int btrfs_find_all_roots(struct btrfs_trans_handle *trans, * callers (such as fiemap) which want to know whether the extent is * shared but do not need a ref count. * - * This attempts to allocate a transaction in order to account for - * delayed refs, but continues on even when the alloc fails. + * This attempts to attach to the running transaction in order to account for + * delayed refs, but continues on even when no running transaction exists. * * Return: 0 if extent is not shared, 1 if it is shared, < 0 on error. */ @@ -1489,8 +1489,12 @@ int btrfs_check_shared(struct btrfs_root *root, u64 inum, u64 bytenr) return -ENOMEM; } - trans = btrfs_join_transaction(root); + trans = btrfs_attach_transaction(root); if (IS_ERR(trans)) { + if (PTR_ERR(trans) != -ENOENT) { + ret = PTR_ERR(trans); + goto out; + } trans = NULL; down_read(&fs_info->commit_root_sem); } else { @@ -1523,6 +1527,7 @@ int btrfs_check_shared(struct btrfs_root *root, u64 inum, u64 bytenr) } else { up_read(&fs_info->commit_root_sem); } +out: ulist_free(tmp); ulist_free(roots); return ret; -- 2.11.0