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/[email protected] [1] Signed-off-by: Christian Brauner (Amutable) <[email protected]> --- Changes in v2: - Use standard naming aligning with other subsystems. - Add a termination record to make this really clean. - Link to v1: https://patch.msgid.link/[email protected] --- Christian Brauner (22): powerpc/spufs: don't dump more than the note supports coredump: refuse negative skips coredump: set the minimum send buffer size selftests/coredump: discard the right amount after the coredump request selftests/coredump: collapse the expected request check into the helper selftests/coredump: add a separate helper header coredump: pin the protocol struct sizes coredump: move the negotiated mask into struct coredump_params coredump: deduplicate the to_skip flush coredump: make the dump helper return bool coredump: always chunk writes coredump: clean up coredump state handling coredump: add COREDUMP_RECORDS to the coredump socket protocol coredump: add COREDUMP_SPARSE to the coredump socket protocol tools: sync coredump.h header coredump: send the coredump in records if requested coredump: describe the holes when COREDUMP_SPARSE is negotiated selftests/coredump: test COREDUMP_RECORDS and COREDUMP_SPARSE selftests/coredump: hand the record stream to a sink selftests/coredump: put a hole in the middle of a sparse mapping selftests/coredump: simulate a blob store selftests/coredump: show how to inspect the task to decide how the coredump should be sent arch/powerpc/platforms/cell/spufs/file.c | 18 +- fs/binfmt_elf.c | 12 +- fs/binfmt_elf_fdpic.c | 12 +- fs/coredump.c | 325 ++++-- include/linux/binfmts.h | 3 +- include/linux/coredump.h | 33 +- include/uapi/linux/coredump.h | 79 +- tools/include/uapi/linux/coredump.h | 79 +- .../coredump/coredump_socket_protocol_test.c | 783 ++++++++++++- tools/testing/selftests/coredump/coredump_test.h | 31 +- .../selftests/coredump/coredump_test_helpers.c | 1171 +++++++++++++++++++- .../selftests/coredump/coredump_test_helpers.h | 53 + 12 files changed, 2399 insertions(+), 200 deletions(-) --- base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f change-id: 20260811-work-coredump-sparse-18177d77b014
