Summarizing what I've seen on the threads...

First of all many thanks for summarizing the info.

1) The bug seems to be read-only snapshot related.  The connection to
send is that send creates read-only snapshots, but people creating read- only snapshots for other purposes are now reporting the same problem, so
it's not send, it's the read-only snapshots.

In fact send does not create a read-only snapshot, snapshots are created manually prior to calling send.

2) Writable snapshots haven't been implicated yet, and the working set
from which the snapshots are taken doesn't seem to be affected, either.
So in that sense it's not affecting ordinary usage, only the read-only
snapshots themselves.

3) More problematic, however, is the fact that these apparently corrupted
read-only snapshots often are not listed properly and can't be deleted,
tho I'm not sure if that's /all/ the corrupted snapshots or only part of
them. So while it may not affect ordinary operation in the short term,
over time until there's a fix, people routinely doing read-only snapshots are going to be getting more and more of these undeletable snapshots, and
depending on whether the eventual patch only prevents more or can
actually fix the bad ones (possibly via btrfs check or the like),
affected filesystems may ultimately have to be blown away and recreated
with a fresh mkfs, in ordered to kill the currently undeletable snapshots.

So the first thing to do would be to shut off whatever's making read-only
snapshots, so you don't make the problem worse while it's being
investigated. For those who can do that without too big an interruption to their normal routine (who don't depend on send/receive, for instance),
just keep it off for the time being.  For those who depend on read-only
snapshots (send-receive for backup and the data is too valuable to not do
the backups for a few days), consider switching back to 3.16-stable --
from 3.16.3 at least, the patch for the compress bug is there, so that
shouldn't be a problem.

And if you're affected, be aware that until we have a fix, we don't know
if it'll be possible to remove the affected and currently undeletable
snapshots.  If it's not, at some point you'll need to do a fresh
mkfs.btrfs, to get rid of the damage.  Since the bug doesn't appear to
affect writable snapshots or the "head" from which snapshots are made,
it's not urgent, and a full fix is likely to include a patch to detect
and fix the problem as well, but until we know what the problem is we
can't be sure of that, so be prepared to do that mkfs at some point, as
at this point it's possible that's the only way you'll be able to kill
the corrupted snapshots.

I don't agree with you concerning the not urgent part. In my opinion, any problem leading to filesystem or other data corruption should be considered as urgent, at least as long as it isn't known what exactly is affected and whether there is a simple way to salvage the corruption without going the backup/restore route.

4) Total speculation on my part, but given the wanted transid (aka
generation, in different contexts) is significantly lower than the found
transid, and the fact that the problem appears to be limited to
/read-only/ snapshots, my first suspicion is that something's getting
updated that would normally apply to all snapshots, but the read-only
nature of the snapshots is preventing the full update there. The transid
of the block is updated, but the snapshot being read-only is preventing
update of the pointer in that snapshot accordingly.

What I do /not/ know is whether the bug is that something's getting
updated that should NOT be, and it's simply the read-only snapshots
letting us know about it since the writable snapshots are fully updated,
even if that breaks the snapshot (breaking writable snapshots in a
different and currently undetected way), or if instead, it's a legitimate
update, like a balance simply moving the snapshot around but not
affecting it otherwise, and the bug is that the read-only snapshots
aren't allowing the legitimate update.

Either way, this more or less developed over the weekend, and it's Monday
now, so the devs should be on it.  If it's anything like the 3.15/3.16
compression bug, it'll take some time for them to properly trace it, and then to figure out an appropriate fix, but they will. Chances are we'll have at least some decent progress on a trace by Friday, and maybe even a
good-to-go patch. =:^)
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