linux-kernel - copy_*

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Date:	Sat, 22 Oct 2011 13:06:06 -0400
From:	Xin Tong <xerox.time.tech@...il.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: copy_*_user

I am investigating copy_from_user  and copy_to_user in linux under
i386. These two function both take a pointer with virtual address and
a pointer with physical address.

copy_from_user calls __copy_from_user_ll and copy_to_user  calls
__copy_to_user_ll. It make sense to me that __copy_to_user_ll converts
the virtual address to physical address using the current process's
page table.

unsigned long __copy_to_user_ll(void __user *to, const void *from,
unsigned long n)
{
   ...
    retval = get_user_pages(current, current->mm,
                                        (unsigned long)to, 1, 1, 0, &pg, NULL);

                        if (retval == -ENOMEM && is_global_init(current)) {
                                up_read(&current->mm->mmap_sem);
                                congestion_wait(BLK_RW_ASYNC, HZ/50);
                                goto survive;
                        }

                        if (retval != 1) {
                                up_read(&current->mm->mmap_sem);
                                break;
                        }

       maddr = kmap_atomic(pg, KM_USER0);
       memcpy(maddr + offset, from, len);
     ...
}

But it seems to be that __copy_from_user_ll  is not converted the
address at all before attempting to copy. Can someone help explain to
me why ?

Thanks
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