On Wed, Jun 15, 2005 at 03:38:45PM +0530, linuxprodigy linuxprodigy wrote:
> I have successfully written to a file through a kernel module. Now
> what I want is to avoid buffer cache while read/write to a file
> through a kernel module. For that I set O_DIRECT flag like shown in
> the code below. But it the write returns error that invalid argument.
> Anybody knows what might the problem n how do I get around with it? (I
> am using kernel 2.6.9).
Please read the mailing list archives on why reading and writing files
from a kernel module is a bad idea.
As for why it doesn't work: direct IO needs page aligned buffers, and
reads and writes need to be done in multiples of the soft block size of
the device.
> Regards,
> Linux Prodigy.
>
> //File od1.c
> #include <linux/kernel.h>
> #include <linux/module.h>
> #include <linux/init.h>
> #include <linux/config.h>
> #include <asm/uaccess.h>
> #include <linux/file.h>
> #include <linux/mm.h>
> #include <linux/slab.h>
>
> static int __init modinit(void)
> {
> struct file *f;
> mm_segment_t orig_fs;
> int lenwrite = 0,i;
> char buff[512],fwrite[] = "/root/programs/odirect/try.txt";
^^^^^^^^^
You only have a *very* limited stack in the kernel, randomly pushing
512 bytes on it is a very bad idea.
> printk(KERN_ALERT "\nHello World 1\n ");
> for(i=0; i<512 ; i++)
> buff[i] = 'x';
Use memset().
> f = filp_open(fwrite,O_WRONLY, 0600);
> f->f_flags |= O_DIRECT;
>
>
> orig_fs = get_fs();
> set_fs(KERNEL_DS);
>
> lenwrite = f->f_op->write(f,buff,512,&f->f_pos);
>
> set_fs(orig_fs);
>
> printk(KERN_ALERT "\nHello World 1 after writing %d \n",lenwrite);
^^
Remove the \n, the point is that klogd recognises the level of the
message because there is a special sequence on the first three lines
of the line. Putting an \n in the message defeats that purpose and
will give your message the standard priority (KERN_INFO, IIRC).
> fput(f);
>
> return 0;
> }
>
> static void __exit modexit(void)
> {
> printk(KERN_ALERT "\nBye World 1 :\n");
> }
>
> module_init(modinit);
> module_exit(modexit);
>
> --
> Kernelnewbies: Help each other learn about the Linux kernel.
> Archive: http://mail.nl.linux.org/kernelnewbies/
> FAQ: http://kernelnewbies.org/faq/
>
--
Erik Mouw
J.A.K.Mouw@xxxxxxxxxxxxxx mouw@xxxxxxxxxxxx
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