Re: [PATCH 2.5.63] Teach page_mapped about the anon flag

[Andrew Morton <akpm@digeo.com>]

Dave McCracken <dmccr@us.ibm.com> wrote:
>
> 
> --On Thursday, February 27, 2003 14:24:50 -0800 Andrew Morton
> <akpm@digeo.com> wrote:
> 
> > I'm just looking at page_mapped().  It is now implicitly assuming that the
> > architecture's representation of a zero-count atomic_t is all-bits-zero.
> > 
> > This is not true on sparc32 if some other CPU is in the middle of an
> > atomic_foo() against that counter.  Maybe the assumption is false on other
> > architectures too.
> > 
> > So page_mapped() really should be performing an atomic_read() if that is
> > appropriate to the particular page.  I guess this involves testing
> > page->mapping.  Which is stable only when the page is locked or
> > mapping->page_lock is held.
> > 
> > It appears that all page_mapped() callers are inside lock_page() at
> > present, so a quick audit and addition of a comment would be appropriate
> > there please.
> 
> I'm not at all confident that page_mapped() is adequately protected.

It is.  All callers which need to be 100% accurate are under
pte_chain_lock().

> Here's a patch that explicitly handles the atomic_t case.

OK..  But it increases dependency on PageAnon.  Wasn't the plan to remove
that at some time?

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