linux.git/fs/btrfs/backref.c, branch v5.3

Btrfs: fix deadlock between fiemap and transaction commits

2019-07-30T16:25:12+00:00

The fiemap handler locks a file range that can have unflushed delalloc,
and after locking the range, it tries to attach to a running transaction.
If the running transaction started its commit, that is, it is in state
TRANS_STATE_COMMIT_START, and either the filesystem was mounted with the
flushoncommit option or the transaction is creating a snapshot for the
subvolume that contains the file that fiemap is operating on, we end up
deadlocking. This happens because fiemap is blocked on the transaction,
waiting for it to complete, and the transaction is waiting for the flushed
dealloc to complete, which requires locking the file range that the fiemap
task already locked. The following stack traces serve as an example of
when this deadlock happens:

  (...)
  [404571.515510] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
  [404571.515956] Call Trace:
  [404571.516360]  ? __schedule+0x3ae/0x7b0
  [404571.516730]  schedule+0x3a/0xb0
  [404571.517104]  lock_extent_bits+0x1ec/0x2a0 [btrfs]
  [404571.517465]  ? remove_wait_queue+0x60/0x60
  [404571.517832]  btrfs_finish_ordered_io+0x292/0x800 [btrfs]
  [404571.518202]  normal_work_helper+0xea/0x530 [btrfs]
  [404571.518566]  process_one_work+0x21e/0x5c0
  [404571.518990]  worker_thread+0x4f/0x3b0
  [404571.519413]  ? process_one_work+0x5c0/0x5c0
  [404571.519829]  kthread+0x103/0x140
  [404571.520191]  ? kthread_create_worker_on_cpu+0x70/0x70
  [404571.520565]  ret_from_fork+0x3a/0x50
  [404571.520915] kworker/u8:6    D    0 31651      2 0x80004000
  [404571.521290] Workqueue: btrfs-flush_delalloc btrfs_flush_delalloc_helper [btrfs]
  (...)
  [404571.537000] fsstress        D    0 13117  13115 0x00004000
  [404571.537263] Call Trace:
  [404571.537524]  ? __schedule+0x3ae/0x7b0
  [404571.537788]  schedule+0x3a/0xb0
  [404571.538066]  wait_current_trans+0xc8/0x100 [btrfs]
  [404571.538349]  ? remove_wait_queue+0x60/0x60
  [404571.538680]  start_transaction+0x33c/0x500 [btrfs]
  [404571.539076]  btrfs_check_shared+0xa3/0x1f0 [btrfs]
  [404571.539513]  ? extent_fiemap+0x2ce/0x650 [btrfs]
  [404571.539866]  extent_fiemap+0x2ce/0x650 [btrfs]
  [404571.540170]  do_vfs_ioctl+0x526/0x6f0
  [404571.540436]  ksys_ioctl+0x70/0x80
  [404571.540734]  __x64_sys_ioctl+0x16/0x20
  [404571.540997]  do_syscall_64+0x60/0x1d0
  [404571.541279]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
  (...)
  [404571.543729] btrfs           D    0 14210  14208 0x00004000
  [404571.544023] Call Trace:
  [404571.544275]  ? __schedule+0x3ae/0x7b0
  [404571.544526]  ? wait_for_completion+0x112/0x1a0
  [404571.544795]  schedule+0x3a/0xb0
  [404571.545064]  schedule_timeout+0x1ff/0x390
  [404571.545351]  ? lock_acquire+0xa6/0x190
  [404571.545638]  ? wait_for_completion+0x49/0x1a0
  [404571.545890]  ? wait_for_completion+0x112/0x1a0
  [404571.546228]  wait_for_completion+0x131/0x1a0
  [404571.546503]  ? wake_up_q+0x70/0x70
  [404571.546775]  btrfs_wait_ordered_extents+0x27c/0x400 [btrfs]
  [404571.547159]  btrfs_commit_transaction+0x3b0/0xae0 [btrfs]
  [404571.547449]  ? btrfs_mksubvol+0x4a4/0x640 [btrfs]
  [404571.547703]  ? remove_wait_queue+0x60/0x60
  [404571.547969]  btrfs_mksubvol+0x605/0x640 [btrfs]
  [404571.548226]  ? __sb_start_write+0xd4/0x1c0
  [404571.548512]  ? mnt_want_write_file+0x24/0x50
  [404571.548789]  btrfs_ioctl_snap_create_transid+0x169/0x1a0 [btrfs]
  [404571.549048]  btrfs_ioctl_snap_create_v2+0x11d/0x170 [btrfs]
  [404571.549307]  btrfs_ioctl+0x133f/0x3150 [btrfs]
  [404571.549549]  ? mem_cgroup_charge_statistics+0x4c/0xd0
  [404571.549792]  ? mem_cgroup_commit_charge+0x84/0x4b0
  [404571.550064]  ? __handle_mm_fault+0xe3e/0x11f0
  [404571.550306]  ? do_raw_spin_unlock+0x49/0xc0
  [404571.550608]  ? _raw_spin_unlock+0x24/0x30
  [404571.550976]  ? __handle_mm_fault+0xedf/0x11f0
  [404571.551319]  ? do_vfs_ioctl+0xa2/0x6f0
  [404571.551659]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
  [404571.552087]  do_vfs_ioctl+0xa2/0x6f0
  [404571.552355]  ksys_ioctl+0x70/0x80
  [404571.552621]  __x64_sys_ioctl+0x16/0x20
  [404571.552864]  do_syscall_64+0x60/0x1d0
  [404571.553104]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
  (...)

If we were joining the transaction instead of attaching to it, we would
not risk a deadlock because a join only blocks if the transaction is in a
state greater then or equals to TRANS_STATE_COMMIT_DOING, and the delalloc
flush performed by a transaction is done before it reaches that state,
when it is in the state TRANS_STATE_COMMIT_START. However a transaction
join is intended for use cases where we do modify the filesystem, and
fiemap only needs to peek at delayed references from the current
transaction in order to determine if extents are shared, and, besides
that, when there is no current transaction or when it blocks to wait for
a current committing transaction to complete, it creates a new transaction
without reserving any space. Such unnecessary transactions, besides doing
unnecessary IO, can cause transaction aborts (-ENOSPC) and unnecessary
rotation of the precious backup roots.

So fix this by adding a new transaction join variant, named join_nostart,
which behaves like the regular join, but it does not create a transaction
when none currently exists or after waiting for a committing transaction
to complete.

Fixes: 03628cdbc64db6 ("Btrfs: do not start a transaction during fiemap")
Signed-off-by: Filipe Manana 
Signed-off-by: David Sterba

btrfs: fiemap: preallocate ulists for btrfs_check_shared

2019-07-01T11:34:53+00:00

btrfs_check_shared looks up parents of a given extent and uses ulists
for that. These are allocated and freed repeatedly. Preallocation in the
caller will avoid the overhead and also allow us to use the GFP_KERNEL
as it is happens before the extent locks are taken.

Reviewed-by: Nikolay Borisov 
Reviewed-by: Filipe Manana 
Signed-off-by: David Sterba

Btrfs: do not start a transaction during fiemap

2019-04-29T17:02:51+00:00

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 
Link: https://lore.kernel.org/linux-btrfs/b2a668d7124f1d3e410367f587926f622b3f03a4.camel@scientia.net/
Fixes: afce772e87c36c ("btrfs: fix check_shared for fiemap ioctl")
CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Qu Wenruo 
Signed-off-by: Filipe Manana 
Signed-off-by: David Sterba

Btrfs: do not start a transaction at iterate_extent_inodes()

2019-04-29T17:02:48+00:00

When finding out which inodes have references on a particular extent, done
by backref.c:iterate_extent_inodes(), from the BTRFS_IOC_LOGICAL_INO (both
v1 and v2) ioctl and from scrub we use the transaction join API to grab a
reference on the currently running transaction, since in order to give
accurate results we need to inspect the delayed references of the currently
running transaction.

However, if there is currently no running transaction, the join operation
will create a new transaction. This is inefficient as the transaction will
eventually be committed, doing unnecessary IO and introducing a potential
point of failure that will lead to a transaction abort due to -ENOSPC, as
recently reported [1].

That's because the join, creates the transaction but does not reserve any
space, so when attempting to update the root item of the root passed to
btrfs_join_transaction(), during the transaction commit, we can end up
failling with -ENOSPC. Users of a join operation are supposed to actually
do some filesystem changes and reserve space by some means, which is not
the case of iterate_extent_inodes(), it is a read-only operation for all
contextes from which it is called.

The reported [1] -ENOSPC failure stack trace is 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 ]---

So fix that by using the attach API, which does not create a transaction
when there is currently no running transaction.

[1] https://lore.kernel.org/linux-btrfs/b2a668d7124f1d3e410367f587926f622b3f03a4.camel@scientia.net/

Reported-by: Zygo Blaxell 
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Filipe Manana 
Reviewed-by: David Sterba 
Signed-off-by: David Sterba

btrfs: use BUG() instead of BUG_ON(1)

2019-04-29T17:02:28+00:00

BUG_ON(1) leads to bogus warnings from clang when
CONFIG_PROFILE_ANNOTATED_BRANCHES is set:

fs/btrfs/volumes.c:5041:3: error: variable 'max_chunk_size' is used uninitialized whenever 'if' condition is false
      [-Werror,-Wsometimes-uninitialized]
                BUG_ON(1);
                ^~~~~~~~~
include/asm-generic/bug.h:61:36: note: expanded from macro 'BUG_ON'
 #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
                                   ^~~~~~~~~~~~~~~~~~~
include/linux/compiler.h:48:23: note: expanded from macro 'unlikely'
 #  define unlikely(x)   (__branch_check__(x, 0, __builtin_constant_p(x)))
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/btrfs/volumes.c:5046:9: note: uninitialized use occurs here
                             max_chunk_size);
                             ^~~~~~~~~~~~~~
include/linux/kernel.h:860:36: note: expanded from macro 'min'
 #define min(x, y)       __careful_cmp(x, y, <)
                                         ^
include/linux/kernel.h:853:17: note: expanded from macro '__careful_cmp'
                __cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
                              ^
include/linux/kernel.h:847:25: note: expanded from macro '__cmp_once'
                typeof(y) unique_y = (y);               \
                                      ^
fs/btrfs/volumes.c:5041:3: note: remove the 'if' if its condition is always true
                BUG_ON(1);
                ^
include/asm-generic/bug.h:61:32: note: expanded from macro 'BUG_ON'
 #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
                               ^
fs/btrfs/volumes.c:4993:20: note: initialize the variable 'max_chunk_size' to silence this warning
        u64 max_chunk_size;
                          ^
                           = 0

Change it to BUG() so clang can see that this code path can never
continue.

Reviewed-by: Nikolay Borisov 
Reviewed-by: David Sterba 
Signed-off-by: Arnd Bergmann 
Signed-off-by: David Sterba

btrfs: honor path->skip_locking in backref code

2019-02-25T13:13:39+00:00

Qgroups will do the old roots lookup at delayed ref time, which could be
while walking down the extent root while running a delayed ref.  This
should be fine, except we specifically lock eb's in the backref walking
code irrespective of path->skip_locking, which deadlocks the system.
Fix up the backref code to honor path->skip_locking, nobody will be
modifying the commit_root when we're searching so it's completely safe
to do.

This happens since fb235dc06fac ("btrfs: qgroup: Move half of the qgroup
accounting time out of commit trans"), kernel may lockup with quota
enabled.

There is one backref trace triggered by snapshot dropping along with
write operation in the source subvolume.  The example can be reliably
reproduced:

  btrfs-cleaner   D    0  4062      2 0x80000000
  Call Trace:
   schedule+0x32/0x90
   btrfs_tree_read_lock+0x93/0x130 [btrfs]
   find_parent_nodes+0x29b/0x1170 [btrfs]
   btrfs_find_all_roots_safe+0xa8/0x120 [btrfs]
   btrfs_find_all_roots+0x57/0x70 [btrfs]
   btrfs_qgroup_trace_extent_post+0x37/0x70 [btrfs]
   btrfs_qgroup_trace_leaf_items+0x10b/0x140 [btrfs]
   btrfs_qgroup_trace_subtree+0xc8/0xe0 [btrfs]
   do_walk_down+0x541/0x5e3 [btrfs]
   walk_down_tree+0xab/0xe7 [btrfs]
   btrfs_drop_snapshot+0x356/0x71a [btrfs]
   btrfs_clean_one_deleted_snapshot+0xb8/0xf0 [btrfs]
   cleaner_kthread+0x12b/0x160 [btrfs]
   kthread+0x112/0x130
   ret_from_fork+0x27/0x50

When dropping snapshots with qgroup enabled, we will trigger backref
walk.

However such backref walk at that timing is pretty dangerous, as if one
of the parent nodes get WRITE locked by other thread, we could cause a
dead lock.

For example:

           FS 260     FS 261 (Dropped)
            node A        node B
           /      \      /      \
       node C      node D      node E
      /   \         /  \        /     \
  leaf F|leaf G|leaf H|leaf I|leaf J|leaf K

The lock sequence would be:

      Thread A (cleaner)             |       Thread B (other writer)
-----------------------------------------------------------------------
write_lock(B)                        |
write_lock(D)                        |
^^^ called by walk_down_tree()       |
                                     |       write_lock(A)
                                     |       write_lock(D) << Stall
read_lock(H) << for backref walk     |
read_lock(D) << lock owner is        |
                the same thread A    |
                so read lock is OK   |
read_lock(A) << Stall                |

So thread A hold write lock D, and needs read lock A to unlock.
While thread B holds write lock A, while needs lock D to unlock.

This will cause a deadlock.

This is not only limited to snapshot dropping case.  As the backref
walk, even only happens on commit trees, is breaking the normal top-down
locking order, makes it deadlock prone.

Fixes: fb235dc06fac ("btrfs: qgroup: Move half of the qgroup accounting time out of commit trans")
CC: stable@vger.kernel.org # 4.14+
Reported-and-tested-by: David Sterba 
Reported-by: Filipe Manana 
Reviewed-by: Qu Wenruo 
Signed-off-by: Josef Bacik 
Reviewed-by: Filipe Manana 
[ rebase to latest branch and fix lock assert bug in btrfs/007 ]
Signed-off-by: Qu Wenruo 
[ copy logs and deadlock analysis from Qu's patch ]
Signed-off-by: David Sterba

btrfs: replace btrfs_set_lock_blocking_rw with appropriate helpers

2019-02-25T13:13:27+00:00

We can use the right helper where the lock type is a fixed parameter.

Reviewed-by: Johannes Thumshirn 
Signed-off-by: David Sterba

btrfs: Fix typos in comments and strings

2018-12-17T13:51:50+00:00

The typos accumulate over time so once in a while time they get fixed in
a large patch.

Signed-off-by: Andrea Gelmini 
Reviewed-by: David Sterba 
Signed-off-by: David Sterba

btrfs: Remove needless tree locking in iterate_inode_extrefs

2018-12-17T13:51:30+00:00

In iterate_inode_exrefs the eb is cloned via btrfs_clone_extent_buffer
which creates a private extent buffer with the dummy flag set and ref
count of 1. Then this buffer is locked for reading and its ref count is
incremented by 1. Finally it's fed to the passed iterate_irefs_t
function. The actual iterate call back is inode_to_path (coming from
paths_from_inode) which feeds the eb to btrfs_ref_to_path. In this final
function the passed eb is only read by first assigning it to the local
eb variable. This variable is only modified in the case another eb was
referenced from the passed path that is eb != eb_in check triggers.

Considering this there is no point in locking the cloned eb in
iterate_inode_refs since it's never being modified and is not published
anywhere. Furthermore the cloned eb is completely fine having its ref
count be 1.

Signed-off-by: Nikolay Borisov 
Reviewed-by: David Sterba 
Signed-off-by: David Sterba

btrfs: Remove needless tree locking in iterate_inode_refs

2018-12-17T13:51:30+00:00

In iterate_inode_refs the eb is cloned via btrfs_clone_extent_buffer
which creates a private extent buffer with the dummy flag set and ref
count of 1. Then this buffer is locked for reading and its ref count is
incremented by 1. Finally it's fed to the passed iterate_irefs_t
function. The actual iterate call back is inode_to_path (coming from
paths_from_inode) which feeds the eb to btrfs_ref_to_path. In this final
function the passed eb is only read by first assigning it to the local
eb variable. This variable is only modified in the case another eb was
referenced from the passed path that is eb != eb_in check triggers.

Considering this there is no point in locking the cloned eb in
iterate_inode_refs since it's never being modified and is not published
anywhere. Furthermore the cloned eb is completely fine having its ref
count be 1.

Signed-off-by: Nikolay Borisov 
Reviewed-by: David Sterba 
Signed-off-by: David Sterba