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Date: Sun, 12 Feb 2006 23:12:23 -0800
From: Andrew Morton <akpm@osdl.org>
To: "David S. Miller" <davem@davemloft.net>
Cc: hugh@veritas.com, wli@holomorphy.com, nickpiggin@yahoo.com.au,
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Subject: Re: [PATCH 1/3] compound page: use page[1].lru
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"David S. Miller" <davem@davemloft.net> wrote:
>
> From: Andrew Morton <akpm@osdl.org>
> Date: Sun, 12 Feb 2006 18:13:12 -0800
> 
> > We have a page which has no ->mapping, but lo, it's mmapped by userspace
> > and can be MAP_SHARED between different CPUs and processes.
> > 
> > Yes, I suspect it'll do the wrong thing in unpleasantly subtle ways.
> > 
> > (cc's davem and runs away).
> 
> 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?)

> Hugepages actually have no D-cache aliasing issues by definition on
> sparc64 because the smallest possible hugepage size is 64K which is
> larger than the D-cache aliasing factor (which is 16K).

OK..  So it would be a bug, except for this hardware quirk.  I'll check the
other architectures.