Re: mmap'ed memory in core files ?

Philippe De Muyter
2008-07-02T11:01:41Z

[ sorry, reposted to add cc linux1394-devel@lists.sourceforge.net ]

Hi Michael,

On Tue, Jul 01, 2008 at 08:16:11PM +0200, Michael Kerrisk wrote:
> On 7/1/08, Philippe De Muyter <phdm@macqel.be> wrote:
> > Hello everybody,
> >
> > I develop video acquisition software using the video1394 interface.
> > The images grabbed by the camera and iee1394 bus are kept in kernel
> > memory and made available to the user program through a mmap call done
> > in the libdc1394 library :
> >
> > dma_ring_buffer= mmap(0, vmmap.nb_buffers * vmmap.buf_size,
> > PROT_READ|PROT_WRITE,MAP_SHARED, craw->capture.dma_fd, 0);
> >
> > Sometimes, my program crashes and produces a core file :) It seems to
> > me that the core file does not contain the mmap'ed memory and hence
> > I cannot replay my program with the same image for debugging purpose.
> >
> > Is it possible to configure the kernel through /proc, or through the mmap
> > system call to have that mmapped segment in the core file, or do I need
> > to modify the kernel itself to obtain the behaviour I want ? If I
> > need to modify the kernel, can some kind soul provide me some pointers ?
>
>
> Have a look at the section "Controlling which mappings are written to
> the core dump" in a recent core.5 man page:
> http://www.kernel.org/doc/man-pages/online/pages/man5/core.5.html

thanks for the info. I didn't know about /proc/PID/coredump_filter.

that part was promising :

bit 2 Dump file-backed private mappings.
bit 3 Dump file-backed shared mappings.

The default value of coredump_filter is 0x3; this reflects traditional
Linux behavior and means that only anonymous memory segments are dumped.

Unfortunately, the part that applies to me (I have tested it) is the next one :

Memory-mapped I/O pages such as frame buffer are never dumped, [...],
regardless of the coredump_filter value.

Is that a design decision, or a mere finding of the way it is implemented
now ?

So, back to my original question :

Can some kind soul provide me some pointers to the way I should modify
the kernel to make the inclusion of the video1394 mmapped segment in
core files possible ?

best regards

Philippe