transfering pages from user space to user space

Dec. 6, 2012

      Hi:
    I am working on a project to copy (page aligned) the buffer content of one process to the buffer of other process.

    Now I resolved this issue using copy_page() but, analizing performance with different buffer sizes, the "copy_page" becames the critical time component and limiting factor.

    I read some articles about, COW,  zero copy and page flipping and I wrote a kernel code to implement page "transfer" (only copy page references), but that code fail and hang. The kernel I use is 2.6.32.

    The pseudocode I think is:

     pte_t *src_pte, *dst_pte;

     src_pte =  getpte(src_task);

     dst_pte =  getpte(dst_task);

     free_pte(dst_pte);    /*to free the page referenced by this PTE */

     *dst_pte = *src_pte; /*copy source PTE to destination PTE */

And here is code added to the kernel:

-------------------------------------------------------------------------------------------------------------

len = PAGE_SIZE;

src_vma=find_vma_intersection(src_proc->p_task->mm,src_addr,src_addr+len);

dst_vma=find_vma_intersection(dst_proc->p_task->mm,dst_addr,dst_addr+len);

src_pgd=pgd_offset(src_proc->p_task->mm, (unsigned long) src_addr);

src_pud=pud_offset(src_pgd,(unsigned long) src_addr);

src_pmd=pmd_offset(src_pud,(unsigned long) src_addr);

src_pte=pte_offset_map_lock(src_proc->p_task->mm, src_pmd,(unsigned long) src_addr, &src_ptl);

dst_pgd=pgd_offset(dst_proc->p_task->mm, (unsigned long) dst_addr);

dst_pud=pud_offset(dst_pgd,(unsigned long) dst_addr);

dst_pmd=pmd_offset(dst_pud,(unsigned long) dst_addr);

dst_pte=pte_offset_map_lock(dst_proc->p_task->mm, dst_pmd,(unsigned long) dst_addr, &dst_ptl);

pte_free(dst_proc->p_task->mm, dst_pte);

copy_pte_range(dst_proc->p_task->mm, src_proc->p_task->mm, dst_pmd, src_pmd, 

                        dst_vma, (unsigned long) dst_addr, ((unsigned long)dst_addr+PAGE_SIZE));

spin_unlock(src_ptl);
spin_unlock(dst_ptl);

-------------------------------------------------------------------------------------------------------------

There are a lots of kernel functions that are not well documented or they have been changed.

Can anybody help me with this issue? Does COW (Copy On Write) will make the copy when the processes will modify their buffers?

I am not suscribed to the mailing list, please CC: the answers to this email account.

Thanks in Advance.

I apologize for my basic English.

Pablo Pessolani

PD: once finishing with that code, some issues about page protection will be considered.