Re: bisected: btrfs dedupe regression in v5.11-rc1: 3078d85c9a10 vfs: verify source area in vfs_dedupe_file_range_one()

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Dec 16, 2021 at 11:29:06PM +0200, Nikolay Borisov wrote:
> 
> 
> On 16.12.21 г. 7:33, Zygo Blaxell wrote:
> > On Wed, Dec 15, 2021 at 12:25:04AM +0200, Nikolay Borisov wrote:
> >> Huhz, this means there is an open transaction handle somewhere o_O. I
> >> checked back the stacktraces in your original email but couldn't see
> >> where that might be coming from. I.e all processes are waiting on
> >> wait_current_trans and this happens _before_ the transaction handle is
> >> opened, hence num_extwriters can't have been incremented by them.
> >>
> >> When an fs wedges, and you get again num_extwriters can you provde the
> >> output of "echo w > /proc/sysrq-trigger"
> > 
> > Here you go...
> 
> <snip>
> 
> > 
> > Again we have "3 locks held" but no list of locks.  WTF is 10883 doing?
> > Well, first of all it's using 100% CPU in the kernel.  Some samples of
> > kernel stacks:
> > 
> > 	# cat /proc/*/task/10883/stack
> > 	[<0>] down_read_nested+0x32/0x140
> > 	[<0>] __btrfs_tree_read_lock+0x2d/0x110
> > 	[<0>] btrfs_tree_read_lock+0x10/0x20
> > 	[<0>] btrfs_search_old_slot+0x627/0x8a0
> > 	[<0>] btrfs_next_old_leaf+0xcb/0x340
> > 	[<0>] find_parent_nodes+0xcd7/0x1c40
> > 	[<0>] btrfs_find_all_leafs+0x63/0xb0
> > 	[<0>] iterate_extent_inodes+0xc8/0x270
> > 	[<0>] iterate_inodes_from_logical+0x9f/0xe0
> 
> That's the real culprit, in this case we are not searching the commit
> root hence we've attached to the transaction. So we are doing backref
> resolution which either:
> 
> a) Hits some pathological case and loops for very long time, backref
> resolution is known to take a lot of time.
> 
> b) We hit a bug in backref resolution and loop forever which again
> results in the transaction being kept open.
> 
> Now I wonder why you were able to bisect this to the seemingly unrelated
> commit in the vfs code.
> 
> Josef any ideas how to proceed further to debug why backref resolution
> takes a long time and if it's just an infinite loop?
> 

It's probably an infinite loop, I'd just start with something like this

bpftrace -e 'tracepoint:btrfs:btrfs_prelim_ref_insert { printf("bytenr is %llu", args->bytenr); }'

and see if it's spitting out the same shit over and over again.  If it is can I
get a btrfs inspect-internal dump-tree -e on the device along with the bytenr
it's hung up on so I can figure out wtf it's tripping over?

If it's not looping there, it may be looping higher up, but I don't see where it
would be doing that.  Lets start here and work our way up if we need to.
Thanks,

Josef