On Mon, Apr 15, 2019 at 02:50:51PM +0100, fdmanana@xxxxxxxxxx wrote: > From: Filipe Manana <fdmanana@xxxxxxxx> > > 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@xxxxxxxxxxxx> > Link: https://lore.kernel.org/linux-btrfs/b2a668d7124f1d3e410367f587926f622b3f03a4.camel@xxxxxxxxxxxx/ > Fixes: afce772e87c36c ("btrfs: fix check_shared for fiemap ioctl") > Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx> > > --- > > 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.
