Re: mmap'ed memory in core files ?

Philippe De Muyter
2008-07-04T11:26:03Z

On Fri, Jul 04, 2008 at 08:33:14AM +0200, Stefan Richter wrote:
> Hidehiro Kawai wrote:
>> Philippe De Muyter wrote:
>>> drivers/ieee1394/dma.c:
>>> int dma_region_mmap(struct dma_region *dma, struct file *file,
>>> struct vm_area_struct *vma)
>>> {
>>> [...]
>>> vma->vm_ops = &dma_region_vm_ops;
>>> vma->vm_private_data = dma;
>>> vma->vm_file = file;
>>> vma->vm_flags |= VM_RESERVED;
>>>
>>> return 0;
>>> }
>>>
>>> So, actually the zone I would like to get dumped in the core file is
>>> tagged
>>> VM_RESERVED.
>>>
>>> I see the following ways to solve my problem :
>>> - do not tag the zone as VM_RESERVED in ieee1394::dma_region_mmap
>>> - tag the zone as VM_ALWAYSDUMP in ieee1394::dma_region_mmap
>>> - add a bit in coredump_filter to dump the VM_RESERVED zones.
>>>
>>> As I don't know the real meaning of VM_RESERVED, I do not know which
>>> choice
>>> is the best one for the official kernel tree,
>
> I don't know these things either. But among else, VM_RESERVED prevents a
> vma from being swapped out. Makes kind of sense, given that besides the
> CPUs also the FireWire controller writes to this memory.
>
>>> but locally I'll go for
>>> adding VM_ALWAYSDUMP in ieee1394::dma_region_mmap.
>> I'm afraid I don't know real usages of VM_RESERVED and VM_IO, either.
>> Allowing everyone to choose whether dump the dma region or not,
>> perhaps we need to introduce a new VM flag (e.g. VM_DUMPABLE) and
>> a coredump_filter bit which controls (VM_IO | VM_RESERVED) &&
>> VM_DUMPABLE area, for example.
>> I think it is also OK to just add VM_ALWAYSDUMP flag to the dma
>> region if the device driver knows the region is safely readable
>> and small enough.
>
> It is safely readable. I don't know if it is small enough. The size of
> the DMA buffer is AFAIK chosen by userspace (by the application program or
> maybe a library) which uses the character device file ABIs for isochronous
> FireWire IO of raw1394, video1394, or dv1394.

For video1394, the size of the buffer is N times the size of a frame, with
N choosen by the application but small (let say < 32). The size of the
frame depends on the camera. I have already worked with 8bpp 1600x1200 cameras,but with a smaller N in that case.

1600x1200x32 gives 60M. If they were allocated by malloc instead of by mmap,
they would be dumped in the core files without any question about the size.

So I think we may consider the 'small enough' criteria is met.

Best regards

Philippe