Re: Doing a zero-copy move of data from a kernel buffer to hard disk

[Neeraj Kumar]

>On 09/20/2010 02:12 PM, Neeraj Kumar wrote:
>> I am trying to move data from a buffer in kernel space into the hard
>> disk without having to incur any additional copies from kernel buffer to
> >user buffers or any other kernel buffers. Any ideas/suggestions would be
> >most helpful.
>>
>>The use case is basically a demux driver which collects data into a
>> demux buffer in kernel space and this buffer has to be emptied
>>periodically by copying the contents into a FUSE-based partition on the
> disk. As the buffer gets full, a user process is signalled which then
>> determines the sector numbers on the disk the contents need to be copied
>> to.
>>
>> I was hoping to mmap the above demux kernel buffer into user address
>> space and issue a write system call to the raw partition device. But
>> from what I can see, the this data is being cached by the kernel on its
>> way to the Hard Disk driver. And so I am assuming that involves
>> additional copies by the linux kernel.
>>
>> At this point I am wondering if there is any other mechansim to do this
>> without involving additional copies by the kernel. I realize this is an
>> unsual usage scenario for non-embedded environments, but I would
>> appreciate any feedback on possible options.
>>
>> BTW - I have tried using O_DIRECT when opening the raw partition, but
>> the subsequent write call fails if the buffer being passed is the
>> mmapped buffer.

>O_DIRECT would be the right way to do it - I think you'd need to figure 
>out why that write is failing. Keep in mind you need to observe the 
>alignment restrictions of O_DIRECT (man 2 open).


I was able to figure out that O_DIRECT can not work with memory mapped 
pages. In the kernel, get_user_pages() returns error if user provided pages are 
mapped kernel pages.

Thanks
Neeraj



      
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