We don't want the assert when we're checking if the backpointer is
valid.

Reported-by: syzbot+bf7215c0525098e7747a@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

if the buffer is virtually mapped it won't be a single bvec

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

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

In backpointer repair, if we get a missing backpointer - but there's
already a backpointer that points to an existing extent - we've got
multiple extents that point to the same space and need to decide which
to keep.

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

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

Remove some duplication, and inconsistency between check_fix_ptrs and
the main ptr marking paths

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

Backpointers that point to invalid devices are caught by fsck, not
.key_invalid; so .key_invalid needs to check for them instead of hitting
asserts.

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

Now that we've got the errors_silent mechanism, we don't have to check
if the reconstruct_alloc option is set all over the place.

Also - users no longer have to explicitly select fsck and fix_errors.

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

Various phases of fsck involve checking references from one btree to
another: this means doing a sequential scan of one btree, and then
mostly random access into the second.

This is particularly painful for checking extents <-> backpointers; we
can prefetch btree node access on the sequential scan, but not on the
random access portion, and this is particularly painful on spinning
rust, where we'd like to keep the pipeline fairly full of btree node
reads so that the elevator can reduce seeking.

This patch implements prefetching and pinning of the portion of the
btree that we'll be doing random access to. We already calculate how
much of the random access btree will fit in memory so it's a fairly
straightforward change.

This will put more pressure on system memory usage, so we introduce a
new option, fsck_memory_usage_percent, which is the percentage of total
system ram that fsck is allowed to pin.

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

This converts -EIOs related to btree node errors to private error codes,
which will help with some ongoing debugging by giving us better error
messages.

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