Re: [PATCH 1/3] compound page: use page[1].lru

["David S. Miller" <davem@davemloft.net>]

From: Andrew Morton <akpm@osdl.org>
Date: Sun, 12 Feb 2006 23:12:23 -0800

> "David S. Miller" <davem@davemloft.net> wrote:
> >
> > The ->mapping check is there essentially to hit user mapped pages that
> > would be modified by the kernel using kernel space memory accesses
> > other than those done by copy_user_page() and clear_user_page() (and
> > their brothers copy_user_highpage() and clear_user_highpage() which
> > just call the former directly on a non-HIGHPAGE platform like
> > sparc64).
> 
> The direct-io.c code just does memset.  (That's very common - maybe
> clear_user_highpage_partial() is needed?)

Yes, something like clear_user_higpage_partial() is definitely needed
for cases like direct-io.c.

We have similar handling for when ptrace() uses get_user_pages() and
tries to write to those pages, via copy_to_user_page() and
copy_from_user_page().  But those interfaces don't clear memory and
they only work in the ptrace path, and they are optimized for that
usage.