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

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

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On Wed, Apr 17, 2019 at 09:22:44AM +0000, Filipe Manana wrote:
> On Wed, Apr 17, 2019 at 1:08 AM Zygo Blaxell
> <ce3g8jdj@umail.furryterror.org> wrote:
> >
> > On Mon, Apr 15, 2019 at 09:29:00AM +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.
> >
> > Cool!
> >
> > Any chance we can do this for the LOGICAL_INO ioctl?  Last time I checked
> > (I admit that was a while ago), LOGICAL_INO cost about the same as
> > fsync(), because it creates transactions with btrfs_join_transaction a
> > few levels deep in the call stack, and gets blocked waiting for them
> > to complete.
> 
> So iterate_extent_inodes() should definitly use the attach API and not join for
> the same reasons. And I can fix that separately.

Uhhh...should it?  I thought there were some callers of
iterate_extent_inodes that definitely do need up-to-date data
from the current transaction for their iterator function...like
maybe record_extent_backrefs?  Then again I'm not sure what
record_extent_backrefs actually does.

> > It looks like btrfs_ioctl_logical_to_ino can set:
> >
> >         path->search_commit_root = 1;
> >
> > just before calling iterate_inodes_from_logical, but I tried that once,
> > and my test filesystem blew up a few days later, so there might be
> > something subtle that I missed.  Or maybe my test filesystem was going
> > to blow up that day anyway--I just assumed that I don't know what I'm
> > doing, and didn't repeat the test.
> 
> Blew up? That's a very vague term... What do you mean with it? What
> happened exactly?

Every few dozen hours of uptime with that change in the test kernel,
btrfs was forcing the filesystem to remount RO.  After a week of that,
there were parent transid verify failures, and my test filesystem stopped
being mountable.

> Setting search_commit_root should not cause any problems, the only
> thing that can happen is giving you non-accurate results
> because there might be relevant changes in the current transaction and
> therefore they are not visible yet from the commit roots.

That's what I thought too, but my one data point said otherwise.  I can
repeat the test if that will be helpful.

> > bees spends about 30% of its time stuck in LOGICAL_INO with a stack
> > trace like this:
> >
> >         [<ffffffffa8400e88>] btrfs_async_run_delayed_refs+0x118/0x140
> >         [<ffffffffa841fbaa>] __btrfs_end_transaction+0x1da/0x2e0
> >         [<ffffffffa848ed19>] iterate_extent_inodes+0x159/0x3a0
> >         [<ffffffffa848eff6>] iterate_inodes_from_logical+0x96/0xb0
> >         [<ffffffffa84590a8>] btrfs_ioctl_logical_to_ino+0xe8/0x190
> >         [<ffffffffa845f1fd>] btrfs_ioctl+0xc5d/0x26a0
> >         [<ffffffffa82a8a62>] do_vfs_ioctl+0x92/0x6b0
> >         [<ffffffffa82a90f4>] SyS_ioctl+0x74/0x80
> >         [<ffffffffa8003b96>] do_syscall_64+0x76/0x180
> >         [<ffffffffa8e00086>] entry_SYSCALL_64_after_hwframe+0x42/0xb7
> >         [<ffffffffffffffff>] 0xffffffffffffffff
> >
> > so if it's at all possible to do LOGICAL_INO without joining a
> > transaction, the benefits should be significant.
> 
> So that is not the same problem. That means that some other task has
> already started a transaction, the transaction
> was not created by the logical_ino ioctl. It just happened that
> logical_ino ended up flushing delayed refs, created by some other
> task(s),
> when releasing its transaction handle.

There is also a lot of time without the delayed_refs call on 4.14 (the
top of the stack is __btrfs_end_transaction).

> We don't flush delayed refs when releasing a transaction handle
> anymore, as of commit:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=db2462a6ad3dc490ac33250042e728226ef3ba00

On 5.0.6 (which has that commit), btrfs_ioctl_logical_to_ino is still
floating between 30% and 40% of the total bees runtime, with a trace like:

	[<0>] wait_current_trans+0xc3/0x100
	[<0>] start_transaction+0x311/0x4d0
	[<0>] iterate_extent_inodes+0x96/0x3c0
	[<0>] iterate_inodes_from_logical+0xa6/0xd0
	[<0>] btrfs_ioctl_logical_to_ino+0xe8/0x190
	[<0>] btrfs_ioctl+0xcf7/0x2cb0
	[<0>] do_vfs_ioctl+0xa2/0x6f0
	[<0>] ksys_ioctl+0x70/0x80
	[<0>] __x64_sys_ioctl+0x16/0x20
	[<0>] do_syscall_64+0x60/0x190
	[<0>] entry_SYSCALL_64_after_hwframe+0x49/0xbe
	[<0>] 0xffffffffffffffff

> > > 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;
> > > +             }
> > >               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
> > >
> 

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