Re: mmap'ed memory in core files ?

[Michael Kerrisk]

[CC+= hidehiro.kawai.ez@xxxxxxxxxxx]

On Wed, Jul 2, 2008 at 12:50 PM, Philippe De Muyter <phdm@xxxxxxxxx> wrote:
> Hi Michael,
>
> On Tue, Jul 01, 2008 at 08:16:11PM +0200, Michael Kerrisk wrote:
>> On 7/1/08, Philippe De Muyter <phdm@xxxxxxxxx> 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
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