Re: [PATCH v6] coredump: Add bit 9 of coredump_filter for pre-exit files before dumping

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/4/26 02:17, Xin Zhao wrote:
> A coredump typically takes seconds or even longer to complete. If we
> happen to hold a write lock with flock just before triggering the
> coredump, that write lock will not be released during the entire coredump
> process. As a result, other processes attempting to acquire the same write
> lock may experience significant delays. Another typical scenario is that
> some custom management modules for shared memory also need to release the
> reference counts of the related buffers as soon as possible, rather than
> waiting until the coredump is complete.
> 
> Add a new bit(9) of coredump_filter to tag whether need to dump fd list.
> We set it by default because tools like systemd-coredump go through the
> fds. Some other coredump pipe programs like minicoredump do not use fds by
> default. If you are sure that your coredump backend does not use the fds,
> you can clear bit 9, which will allow some file resources without VMA
> references to be released earlier.
> 
> In fput(), check FP_DUMPCORE task flags to NOT release file by task work,
> otherwise file put operation will NOT execute util coredump finish.
> 
> Test Case One - flock
> Test program send signal SIGABRT to the program which owns the flock,
> output the wait time(unit ms) to successfully attach the flock.
> Test program malloc 500MB heap and memset it.
> If NOT set bit9 of coredump_filter, waitms is 11280.
> If set bit9 of coredump_filter, waitms is 0.
> 
> Test Case Two - ion buffer
> Test programs include ion buffer publisher and ion buffer subscriber.
> Ion buffer publisher output the ion buffer hold_time if the subscriber
> NOT send ack to publisher and NOT release it. The subscriber will trig
> coredump by itself in some time.
> If NOT set bit9 of subscriber coredump_filter, max hold_time is 19591ms.
> If set bit9 of subscriber coredump_filter, max hold_time is 320ms.
> 
> Signed-off-by: Xin Zhao <jackzxcui1989@163.com>
> ---
> 
> Change in v6:
> - Fix operator precedence in PF_KTHREAD/PF_DUMPCORE check.
> 
> Change in v5:
> - Not add another bootargs for the feature,
>   as suggested by Christian Brauner and Lorenzo Stoakes.
>   Add bit9 of coredump_filter to tag whether need to dump fd list.
>   Set bit9 to 1 as default.
> - Al Viro, Christian Brauner and Lorenzo Stoakes point out so many
>   problems of the code related to umap that was added in v4, delete all of
>   it which is unnecessary. The management of reference counting for shared
>   memory generally does not need to be released through the release
>   operation of files that have VMA references. Traversing all the threads
>   within the process and executing exit_files() is sufficient.
> - Fulfill comments and commit log,
>   as suggested by Pedro Falcato and Lorenzo Stoakes.
> - Link to v5: https://lore.kernel.org/all/20260630075604.52533-1-jackzxcui1989@163.com/
> 
> Change in v4:
> - Christian pointed out that the coredump process will traverse file
>   descriptors (fd), so certain fds should not be closed by default.
>   Rework the whole feature, add /proc/<pid>/coredump_pre_exit for user
>   pre-exit resources selection, default is NOT pre-exit anything.
> - Mateusz suggested that walking the fd table and release the file-lock is
>   reasonable. No longer release all the fd(s). Based on user config, only
>   the flock fd(s) and the fd(s) correspondent to file-backed shared memory
>   will be released at most.
> - Link to v4: https://lore.kernel.org/all/20260624145552.70143-1-jackzxcui1989@163.com/
> 
> Change in v3:
> - Add comment and commit-log to explain why do the MMF_DUMP_MAPPED_SHARED
>   mm_flags_test() check, note that memory mapped files keep their own
>   separate references to the files. The case to work around is that early
>   unlocking a flock on a file allows other processes to lock and modify
>   the mapped data protected by the flock,
>   as suggested by Pedro Falcato.
> - Link to v3: https://lore.kernel.org/all/20260619122419.3954581-1-jackzxcui1989@163.com/
> 
> Change in v2:
> - Get rid of the implement of adding new fcntl API, the issue does not
>   worth inflicting the cost on everyone,
>   as suggested by Al Viro.
> - Call exit_files() in coredump_wait(),
>   as suggested by Eric W. Biederman.
>   Add MMF_DUMP_MAPPED_SHARED mm_flags_test() check to filter cases that
>   need to dump file-backed shared memory.
> - Link to v2: https://lore.kernel.org/lkml/20260618150301.3226517-1-jackzxcui1989@163.com/
> 
> v1:
> - Link to v1: https://lore.kernel.org/all/20260618030700.2511668-1-jackzxcui1989@163.com/
> ---
>  Documentation/filesystems/proc.rst | 14 ++++++++++++--
>  fs/coredump.c                      | 21 +++++++++++++++++++++
>  fs/file_table.c                    |  7 ++++++-
>  include/linux/mm_types.h           |  6 ++++--
>  4 files changed, 43 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index db6167bef..d590a1dda 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1939,6 +1939,7 @@ The following 9 memory types are supported:
>    - (bit 6) hugetlb shared memory
>    - (bit 7) DAX private memory
>    - (bit 8) DAX shared memory
> +  - (bit 9) fd list

"The following 9 memory types are supported: ... fd list"

What?

-- 
Cheers,

David