Re: mmap'ed memory in core files ?

Stefan Richter
2008-07-04T06:34:15Z

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.
--
Stefan Richter
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