On Mon, Apr 15, 2019 at 9:45 AM Qu Wenruo <[email protected]> wrote:
>
>
>
> On 2019/4/15 下午4:29, [email protected] wrote:
> > From: Filipe Manana <[email protected]>
> >
> > 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 <[email protected]>
> > Link:
> > https://lore.kernel.org/linux-btrfs/[email protected]/
> > Fixes: afce772e87c36c ("btrfs: fix check_shared for fiemap ioctl")
> > Signed-off-by: Filipe Manana <[email protected]>
> > ---
> > 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.
So?
If an extent is not shared but right before btrfs_check_shared()
returns it becomes shared? We will report it as not shared.
It's the same type of "problem".
>
> 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;
> >
>
