large copy_to_user fills only one page?

[Emmanuel Michon <emmanuel_michon@realmagic.fr>]

Hi,

I'm working with linux-2.4.18, and writing some
trivial code to get from kernel a grabbed image working this way:

#define IMSIZE 350000

user mode runs:
u_p=malloc(IMSIZE);
ioctl(grabberfd,DOGRAB,u_p);
write *u_p to disk
free(u_p);

kernelmode runs:
case DOGRAB:
        char *u_p,*k_p;
        copy_from_user(u_p,arg,sizeof(char *));
        k_p=vmalloc(IMSIZE);
        kernelgrabs(k_p);
        copy_to_user(u_p,k_p,IMSIZE);
        vfree(k_p);
        break;

What I get actually is only 4K filled in userland, but copy_to_user
returns IMSIZE!

If I memset the memory area *u_p to any value, the grab happens
properly.

I guess memset'ing faults the good pages in, I'm quite surprised
this does not happen smoothly by itself ;-( 

Any clue?

Sincerely yours,

-- 
Emmanuel Michon
Chef de projet
REALmagic France SAS
Mobile: 0614372733 GPGkeyID: D2997E42