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

[Blaisorblade <blaisorblade@yahoo.it>]

On Tuesday 02 May 2006 05:53, Nick Piggin wrote:
> 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.

What could be done is to set a PTE with "no protection", use another bit 
rather than _PAGE_PROTNONE. This wastes one more bit but doable.

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

I must think on this, but the semantics are not entirely the same between the 
two cases. You have no page attached when _PAGE_FILE is there, but a page is 
attached to the PTE with only _PAGE_PROTNONE. Testing that via VM_MANYPROTS 
is just as slow as-is (can be changed with code duplication for the linear 
and non-linear cases).

The application semantics can also be different when you remap as read/write 
that page - the app could have stored an offset there (this is less definite 
since you can't remap & keep the offset currently).

Also, this wouldn't solve the problem, it would make the solution harder: how 
do I know that there's no page to call page_remove_rmap() on, without 
_PAGE_FILE?

I thought to change _PAGE_PROTNONE: it is used to hold a page present and 
referenced but unaccessible. It seems it could be released when 
_PAGE_PROTNONE is set, but for anonymous memory it's impossible. When I've 
asked Hugh about this, he imagined the case when an application faults in a 
page in a VMA then mprotects(PROT_NONE) it; the PTE is set as PROT_NONE. We 
can avoid that in the VM_MAYSHARE case (VM_SHARED or PROT_SHARED was set but 
the file is readonly), but not when anonymous memory is present - the 
application could want it back.

> > 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.

Ok, only unlikely for test on _PAGE_PROTNONE and ! _PAGE_FILE.
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade
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