Re: [patch 11/14] remap_file_pages protection support: pte_present should not trigger on PTE_FILE PROTNONE ptes

[Nick Piggin <nickpiggin@yahoo.com.au>]

blaisorblade@yahoo.it wrote:
> From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
> 
> pte_present(pte) implies that pte_pfn(pte) is valid. Normally even with a
> _PAGE_PROTNONE pte this holds, but not when such a PTE is installed by
> the new install_file_pte; previously it didn't store protections, only file
> offsets, with the patches it also stores protections, and can set
> _PAGE_PROTNONE|_PAGE_FILE.

Why is this combination useful? Can't you just drop the _PAGE_FILE from
_PAGE_PROTNONE ptes?

> 
> zap_pte_range, when acting on such a pte, calls vm_normal_page and gets
> &mem_map[0], does page_remove_rmap, and we're easily in trouble, because it
> happens to find a page with mapcount == 0. And it BUGs on this!
> 
> I've seen this trigger easily and repeatably on UML on 2.6.16-rc3. This was
> likely avoided in the past by the PageReserved test - page 0 *had* to be
> reserved on i386 (dunno on UML).
> 
> Implementation follows for UML and i386.
> 
> To avoid additional overhead, I also considered adding likely() for
> _PAGE_PRESENT and unlikely() for the rest, but I'm uncertain about validity of
> possible [un]likely(pte_present()) occurrences.

Not present pages are likely to be pretty common when unmapping.

I don't like this patch much.

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