On Fri, Aug 3, 2018 at 10:13 AM, robbieko <robbieko@xxxxxxxxxxxx> wrote:
> From: Robbie Ko <robbieko@xxxxxxxxxxxx>
>
> Commit e9894fd3e3b3 ("Btrfs: fix snapshot vs nocow writting")
> forced writeback fallback to COW when subvolume is snapshotted.
Commit e9894fd3e3b3 ("Btrfs: fix snapshot vs nocow writting") forced
nocow writes to fallback
to COW, during writeback, when a snapshot is created. This resulted in
writes made before creating
the snapshot to unexpectedly fail with ENOSPC during writeback when
success (0) was returned
to user space through the write system call.
The steps leading to this problem are:
>
> 1. When the space is full, write syscall will check if can
> nocow, and space reservation will not happen.
1. When it's not possible to allocate data space for a write, the
buffered write path checks if
a NOCOW write is possible. If it is, it will not reserve space and
success (0) is returned to
user space.
>
> 2. Then snapshot happens before flushing IO (running dealloc),
> we will increase will_be_snapshotted, and then when running
> dealloc we fallback to COW and fail (ENOSPC).
2. Then when a snapshot is created, the root's will_be_snapshotted
atomic is incremented and writeback
is triggered for all inode's that belong to the root being
snapshotted. Incrementing that atomic forces
all previous writes to fallback to COW during writeback (running delalloc).
3. This results in the writeback for the inodes to fail and therefore
setting the ENOSPC error in their mappings,
so that a subsequent fsync on them will report the error to user
space. So it's not a completely silent data loss
(since fsync will report ENOSPC) but it's a very unexpected and
undesirable behaviour, because if a clean
shutdown/unmount of the filesystem happens without previous calls to
fsync, it is expected to have the data
present in the files after mounting the filesystem again.
>
> So fix this by we add a snapshot_force_cow, this is used to
> distinguish between write and writeback.
So fix this by adding a new atomic named snapshot_force_cow to the
root structure which prevents
this behaviour and works the following way:
>
> 1. Increase will_be_snapshotted, so that write force to the cow,
> always need space reservation.
1. It is incremented when we start to create a snapshot after
triggering writeback and
before waiting for writeback to finish.
>
> 2. Flushing all dirty pages (running dealloc), then now writeback
> is still flushed in nocow mode, make sure all ditry pages that might
> not reserve space previously have flushed this time otherwise they
> will fallback to cow mode and fail due to no space.
2. This new atomic is now what is used by writeback (running delalloc)
to decide whether we need to
fallback to COW or not. Because we incremented this new atomic after
triggering writeback in the snapshot
creation ioctl, we ensure that all buffered writes that happened
before snapshot creation will succeed and
not fallback to COW (which would make them fail with ENOSPC).
>
> 3. Increase snapshot_force_cow, since all new dirty pages are
> guaranteed space reservation, when running dealloc we can safely
> fallback to COW.
3. The existing atomic, will_be_snapshotted, is kept because it is
used to force new buffered writes, that
start after we started snapshotting, to reserve data space even when
NOCOW is possible.
This makes these writes fail early with ENOSPC when there's no
available space to allocate, preventing the
unexpected behaviour of writeback later failing with ENOSPC due to a
fallback to COW mode.
>
> Fixes: e9894fd3e3b3 ("Btrfs: fix snapshot vs nocow writting")
> Signed-off-by: Robbie Ko <robbieko@xxxxxxxxxxxx>
> ---
> fs/btrfs/ctree.h | 1 +
> fs/btrfs/disk-io.c | 1 +
> fs/btrfs/inode.c | 26 +++++---------------------
> fs/btrfs/ioctl.c | 14 ++++++++++++++
> 4 files changed, 21 insertions(+), 21 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 118346a..663ce05 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1277,6 +1277,7 @@ struct btrfs_root {
> int send_in_progress;
> struct btrfs_subvolume_writers *subv_writers;
> atomic_t will_be_snapshotted;
> + atomic_t snapshot_force_cow;
>
> /* For qgroup metadata reserved space */
> spinlock_t qgroup_meta_rsv_lock;
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 205092d..5573916 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1216,6 +1216,7 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
> atomic_set(&root->log_batch, 0);
> refcount_set(&root->refs, 1);
> atomic_set(&root->will_be_snapshotted, 0);
> + atomic_set(&root->snapshot_force_cow, 0);
> root->log_transid = 0;
> root->log_transid_committed = -1;
> root->last_log_commit = 0;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index eba61bc..263b852 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -1275,7 +1275,7 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> u64 disk_num_bytes;
> u64 ram_bytes;
> int extent_type;
> - int ret, err;
> + int ret;
> int type;
> int nocow;
> int check_prev = 1;
> @@ -1407,11 +1407,9 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> * if there are pending snapshots for this root,
> * we fall into common COW way.
> */
> - if (!nolock) {
> - err = btrfs_start_write_no_snapshotting(root);
> - if (!err)
> - goto out_check;
> - }
> + if (!nolock &&
> + unlikely(atomic_read(&root->snapshot_force_cow)))
> + goto out_check;
> /*
> * force cow if csum exists in the range.
> * this ensure that csum for a given extent are
> @@ -1420,9 +1418,6 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> ret = csum_exist_in_range(fs_info, disk_bytenr,
> num_bytes);
> if (ret) {
> - if (!nolock)
> - btrfs_end_write_no_snapshotting(root);
> -
> /*
> * ret could be -EIO if the above fails to read
> * metadata.
> @@ -1435,11 +1430,8 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> WARN_ON_ONCE(nolock);
> goto out_check;
> }
> - if (!btrfs_inc_nocow_writers(fs_info, disk_bytenr)) {
> - if (!nolock)
> - btrfs_end_write_no_snapshotting(root);
> + if (!btrfs_inc_nocow_writers(fs_info, disk_bytenr))
> goto out_check;
> - }
> nocow = 1;
> } else if (extent_type == BTRFS_FILE_EXTENT_INLINE) {
> extent_end = found_key.offset +
> @@ -1453,8 +1445,6 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> out_check:
> if (extent_end <= start) {
> path->slots[0]++;
> - if (!nolock && nocow)
> - btrfs_end_write_no_snapshotting(root);
> if (nocow)
> btrfs_dec_nocow_writers(fs_info, disk_bytenr);
> goto next_slot;
> @@ -1476,8 +1466,6 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> end, page_started, nr_written, 1,
> NULL);
> if (ret) {
> - if (!nolock && nocow)
> - btrfs_end_write_no_snapshotting(root);
> if (nocow)
> btrfs_dec_nocow_writers(fs_info,
> disk_bytenr);
> @@ -1497,8 +1485,6 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> ram_bytes, BTRFS_COMPRESS_NONE,
> BTRFS_ORDERED_PREALLOC);
> if (IS_ERR(em)) {
> - if (!nolock && nocow)
> - btrfs_end_write_no_snapshotting(root);
> if (nocow)
> btrfs_dec_nocow_writers(fs_info,
> disk_bytenr);
> @@ -1537,8 +1523,6 @@ static noinline int run_delalloc_nocow(struct inode *inode,
> EXTENT_CLEAR_DATA_RESV,
> PAGE_UNLOCK | PAGE_SET_PRIVATE2);
>
> - if (!nolock && nocow)
> - btrfs_end_write_no_snapshotting(root);
> cur_offset = extent_end;
>
> /*
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index b077544..42af06b 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -761,6 +761,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
> struct btrfs_pending_snapshot *pending_snapshot;
> struct btrfs_trans_handle *trans;
> int ret;
> + bool snapshot_force_cow = false;
>
> if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))
> return -EINVAL;
> @@ -777,6 +778,10 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
> goto free_pending;
> }
>
> + /*
> + * We force a new write to reserve space to
> + * avoid the space being full since they'll fallback to cow.
> + */
Force new buffered writes to reserve space even when NOCOW is
possible. This is to
avoid later writeback (running dealloc) to fallback to COW mode and
unexpectedly fail
with ENOSPC.
> atomic_inc(&root->will_be_snapshotted);
> smp_mb__after_atomic();
> /* wait for no snapshot writes */
> @@ -787,6 +792,13 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
> if (ret)
> goto dec_and_free;
>
> + /*
> + * When all previous writes are finished,
> + * we can safely convert writeback to cow.
> + */
All previous writes have started writeback in NOCOW mode, so now we
force future writes to
fallback to COW mode during snapshot creation.
I would also change the subject from:
"Btrfs: optimization to avoid ENOSPC for nocow writes after snapshot
when low on data space"
to:
"Btrfs: fix unexpected failure of nocow buffered writes after
snapshotting when low on space"
With that you can have my:
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Thanks, great work!
> + atomic_inc(&root->snapshot_force_cow);
> + snapshot_force_cow = true;
> +
> btrfs_wait_ordered_extents(root, U64_MAX, 0, (u64)-1);
>
> btrfs_init_block_rsv(&pending_snapshot->block_rsv,
> @@ -851,6 +863,8 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
> fail:
> btrfs_subvolume_release_metadata(fs_info, &pending_snapshot->block_rsv);
> dec_and_free:
> + if (snapshot_force_cow)
> + atomic_dec(&root->snapshot_force_cow);
> if (atomic_dec_and_test(&root->will_be_snapshotted))
> wake_up_var(&root->will_be_snapshotted);
> free_pending:
> --
> 1.9.1
>
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--
Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”
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