Merge patch series "coredump: allow to create sparse coredumps on the coredump socket"

Christian Brauner <brauner@kernel.org> says:

A coredump generated via the coredump socket ends up transferring
zeroed data when a mapping contains holes. For a large process that
maps a bunch of data that's wasting a ton of work.

Jacob ran into this and Josef has bitched^wcomplained about this to me
before. I dislike the coredump_filter bit solution in [1] which stops
each PT_LOAD at the last populated page.

The problem is real though. I don't think coredump_filter is where we
need to solve this. That mask says which kinds of memory to include and
it propagates across fork and exec, whereas what is being selected here
is an encoding mechanism.

I also think that the usermodehelper - may it swiftly die - isn't really
salvagable for this and it's not the future anyway. The coredump socket
already has a handshake for stuff like this.

I always had an idea how this would look like but punted on it back
then. So here it is.

A server that raises COREDUMP_RECORDS in coredump_ack->mask doesn't get
the coredump as a plain byte stream but as a sequence of records. Each
one a struct coredump_record_header followed by what it describes. A
data record carries its bytes. If a server also raises COREDUMP_SPARSE,
zero records are sent for unpopulated mappings. They only indicate how
many zero bytes need to be written and do not include data. Reassembling
the records gives back the same coredump. A debugger and everything else
still see an ordinary core file and nothing outside the coredump server
has to learn anything.

Numbers from the selftests, on a kernel built from this series:

- a process with 128 threads: 1424153 bytes on the socket for a
  coredump of 1075150848 bytes
- a 256MB mapping with the first and last page touched: 188793 bytes on
  the socket for a coredump of 268890112 bytes
- the same 256MB mapping with COREDUMP_RECORDS alone: 271009312 bytes on
  the socket, so the record overhead itself is under one percent

The first one is the interesting case. Almost all of it is thread stacks.
All stacks are 8MB reservations that are nearly all holes. And they are
holes in the middle of the dump rather than at the end.

Link: https://lore.kernel.org/all/20260731171336.2255844-1-jalalonde@meta.com [1]

* patches from https://patch.msgid.link/20260820-work-coredump-sparse-v2-0-ba32dd718c51@kernel.org: (21 commits)
  selftests/coredump: show how to inspect the task to decide how the coredump should be sent
  selftests/coredump: simulate a blob store
  selftests/coredump: put a hole in the middle of a sparse mapping
  selftests/coredump: hand the record stream to a sink
  selftests/coredump: test COREDUMP_RECORDS and COREDUMP_SPARSE
  coredump: describe the holes when COREDUMP_SPARSE is negotiated
  coredump: send the coredump in records if requested
  tools: sync coredump.h header
  coredump: add COREDUMP_SPARSE to the coredump socket protocol
  coredump: add COREDUMP_RECORDS to the coredump socket protocol
  coredump: clean up coredump state handling
  coredump: always chunk writes
  coredump: make the dump helper return bool
  coredump: deduplicate the to_skip flush
  coredump: move the negotiated mask into struct coredump_params
  coredump: pin the protocol struct sizes
  selftests/coredump: add a separate helper header
  selftests/coredump: collapse the expected request check into the helper
  selftests/coredump: discard the right amount after the coredump request
  coredump: set the minimum send buffer size
  ...

Link: https://patch.msgid.link/20260820-work-coredump-sparse-v2-0-ba32dd718c51@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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