Re: [PATCH] zerocopy NFS updated

[Hirokazu Takahashi]

Hi, David

davem>    Now I wonder if we could make these pages COW mode.
davem>    When some process try to update the pages, they should be duplicated.
davem>    I's easy to implement it in write(), truncate() and so on.
davem>    But mmap() is little bit difficult if there no reverse mapping page to PTE.
davem>    
davem>    How do you think about this idea?
davem> 
davem> I think this idea has such high overhead that it is even not for
davem> consideration, consider SMP.

Hmmm... If I'd implement them.....
How about following codes ?

nfsd read()
{
	   :
    page_cache_get(page);
    if (page is mapped to anywhere)
        page = duplicate_and_rehash(page);
    else {
        page_lock(page);
	page->flags |= COW;
	page_unlock(page);
    }
    sendpage(page);
    page_cache_release(page);
}

generic_file_write()
{
    page = _grab_cache_page()
    lock_page(page);
    if (page->flags & COW)
        page = duplicate_and_rehash(page);
    prepare_write();
    commit_write();
    UnlockPage(page);
    page_cache_release(page)
}

truncate_list_page()  <-- truncate() calls
{
    page_cache_get();
    lock_page(page);
    if (page->flags & COW)
        page = duplicate_and_rehash(page);
    truncate_partial_page();
    UnlockPage(page);
    page_cache_release(page);
}