Add a tracepoint for any time we return an error and unwind.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Currently, device removal has to scan all metadata for pointers to the
device being removed.

Add a new method, with the same interface as bch2_dev_data_drop(), that
scans by backpointers instead - this will drastically speed up device
removal.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

This was planned to be done ages ago, now finally completed; there are
places where we have quite a few btree_trans objects on the stack, so
this reduces stack usage somewhat.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

This code needs quite a bit of work: we don't want to be walking all
metadata in the filesystem, we should just be walking backpointers, and
it should be switched to a data ioctl that can report progress via a
file descriptor, not the system console.

But that'll take more work - before we can safely walk only backpointers
we need to change device add to not reuse device indexes, since with
that change accounting being wrong introduces the possibility of
removing a device that still has pointers.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Combine iter/update/trigger/str_hash flags into a single enum, and
x-macroize them for a to_text() function later.

These flags are all for a specific iter/key/update context, so it makes
sense to group them together - iter/update/trigger flags were already
given distinct bits, this cleans up and unifies that handling.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Repurposing standard error codes in bcachefs code is banned in new code,
and we need to get rid of the remaining ones - private error codes give
us much better error messages.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

BTREE_INSERT flags are actually transaction commit flags - rename them
for clarity.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

We're using more stack than we'd like in a number of functions, and
btree_trans is the biggest object that we stack allocate.

But we have to do a heap allocatation to initialize it anyways, so
there's no real downside to heap allocating the entire thing.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>