From: Qu Wenruo <quwenruo.btrfs@gmx.com>
To: fdmanana@kernel.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] Btrfs: do not start a transaction during fiemap
Date: Mon, 15 Apr 2019 16:45:32 +0800 [thread overview]
Message-ID: <3b22cc0c-7152-8345-e766-d439e9c34d00@gmx.com> (raw)
In-Reply-To: <20190415082900.2023-1-fdmanana@kernel.org>
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On 2019/4/15 下午4:29, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
>
> 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 <calestyo@scientia.net>
> 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 <fdmanana@suse.com>
> ---
> 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;
> + }
My concern is, if at this timing there is no running transaction, so we
continue with trans == NULL.
But before we continue, some one started a transaction and
increased/decreased the extent reference number, this doesn't look as safe.
Or did I miss something?
Thanks,
Qu
> 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;
>
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next prev parent reply other threads:[~2019-04-15 8:45 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-15 8:29 [PATCH] Btrfs: do not start a transaction during fiemap fdmanana
2019-04-15 8:45 ` Qu Wenruo [this message]
2019-04-15 8:51 ` Filipe Manana
2019-04-15 9:03 ` Qu Wenruo
2019-04-15 13:50 ` [PATCH v2] " fdmanana
2019-04-23 12:10 ` David Sterba
2019-04-17 0:07 ` [PATCH] " Zygo Blaxell
2019-04-17 9:22 ` Filipe Manana
2019-04-17 9:35 ` David Sterba
2019-04-17 16:50 ` Zygo Blaxell
2019-04-17 17:40 ` Traces during logical_ino ioctl, slowdown, etc (was Re: [PATCH] Btrfs: do not start a transaction during fiemap) Filipe Manana
2019-04-17 20:44 ` Zygo Blaxell
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