Re: [PATCH v2] Btrfs: do not start a transaction during fiemap

From: David Sterba <dsterba@suse.cz>
To: fdmanana@kernel.org
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v2] Btrfs: do not start a transaction during fiemap
Date: Tue, 23 Apr 2019 14:10:30 +0200	[thread overview]
Message-ID: <20190423121030.GU20156@twin.jikos.cz> (raw)
In-Reply-To: <20190415135051.28076-1-fdmanana@kernel.org>

On Mon, Apr 15, 2019 at 02:50:51PM +0100, 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>
> 
> ---
> 
> V2: Consider EROFS due to a readonly state after some transaction abort, to allow
>     fiemap since it's a readonly operation. Converted first error return to a goto
>     to the introduced label.

Added to misc-next, thanks.