Re: [PATCH] coredump/fcntl: Add FD_CLOBCOR flag to close fd before dumping core

[Eric W. Biederman]

Xin Zhao <jackzxcui1989@xxxxxxx> writes:

> A coredump typically takes some time 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.

You are talking about giant processes writing to slow backing store?

I suspect you would be better off quickly writing the coredump to a pipe,
and then writing it to disk.

Unless your machine is badly balanced that should take perhaps a second.

That said I don't see why you need elaborate machinery to do something
about these file descriptors.  Unless I am mistaken no file descriptors
are placed into a coredump.  In which case it should be possible to just
call exit_files early.

> To address this, we introduce the F_[GET|SET]FD_EX fcntl operation and the
> FD_CLOBCOR flag, allowing coredump_wait() to release any file descriptors
> marked with FD_CLOBCOR. We can also assign the FD_CLOBCOR flag to specific
> shared memory segments, preventing the coredump from including shared
> memory that we are not interested in, thereby reducing both the coredump
> duration and the size of the core file.

Please look at vma_dump_size.  There are plenty of ways already to
skip dumping a memory area.  Using file backed shared memory,
and madvise(MADV_DONTDUMP) are two easy ones that already exist.

My point is that there are cleaner ways to solve your problem than
the solutions you have proposed.

Eric