I found that O_DIRECT calls get_user_pages() to map kernel user space pages in order to provide direct user-space access to the driver.
get_user_pages cheks thats if the user space address is memory-mapped or not by VM_IO flag.
If it is memory mapped then it retuns "Bad Address" error.
if ( !vma
|| (pages && vma->vm_flags & VM_IO) /*
|| !(flags & vma->vm_flags)) // I removed this line and O_DIRECT works now for memory mapped address
{ printk ( "FAILING here 1");
return i ? : -EFAULT;
}
But now sure how much correct this hack is???
-manisha
On 7/4/06, Learner <ruxyz@xxxxxxxxx> wrote:
Check out this
http://oss.sgi.com/archives/xfs/2003-05/msg00082.html
--- Manisha Maheshwari < maheshwari.manisha@xxxxxxxxx>
wrote:
> I am having problem reading file if the file is
> opened with O_DIERCT
> flag.The error given by read system call is 'Bad
> Address'.
> The buffer where i am trying to read is a device
> that is mapped into the
> process address space.
> I have checked that buffer is mapped by looking into
> the proc/<pid of my
> process>/maps.
> I am using ext2 filesysytem with blocksize is 2048.
> The address where i am trying to read is block
> aligned(i.e 2048
> byte aligned) and the number of bytes i am trying to
> read is also multiple
> of 2048.
> I am using kernel release:2.6.XXX.
>
> I couldn't understand why it's creating problem with
> O_DIRECT because the
> program works if i open file without O_DIRECT.
>
> -manisha
>
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