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 wrote:
> 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.

I see.

> 
> 
>>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.

And yes, this won't work. I was misunderstanding what was happening.

I guess your problem is that you're overloading the pte protection bits
for present ptes as protection bits for not present (file) ptes. I'd rather
you just used a different encoding for file pte protections then.

"Wasting" a bit seems much more preferable for this very uncommon case (for
most people) rather than bloating pte_present check, which is called in
practically every performance critical inner loop).

That said, if the patch is i386/uml specific then I don't have much say in
it. If Ingo/Linus and Jeff/Yourself, respectively, accept the patch, then
fine.

But I think you should drop the comment from the core code. It seems wrong.

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