Re: mmap'ed memory in core files ?

Michael Kerrisk
2008-07-02T10:58:18Z

[CC+= hidehiro.kawai.ez@hitachi.com]

On Wed, Jul 2, 2008 at 12:50 PM, Philippe De Muyter <phdm@macqel.be> wrote:
> 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 ?

Perhaps Hidehiro, who wrote the coredump_filter feature, can provide insight.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html