Re: [PATCH 2.6.21] cramfs: add cramfs Linear XIP

[Jared Hulbert]

On 6/15/07, Carsten Otte <cotte@xxxxxxxxxx> wrote:
> Nick Piggin wrote:
> > Carsten Otte wrote:
> >> The current xip stack relies on having struct page behind the memory
> >> segment. This causes few impact on memory management, but occupies
> >> some more memory. The cramfs patch chose to modify copy on write in
> >> order to deal with vmas that don't have struct page behind.
> >> So far, Hugh and Linus have shown strong opposition against copy on
> >> write with no struct page behind. If this implementation is acceptable
> >> to the them, it seems preferable to me over wasting memory. The xip
> >> stack should be modified to use this vma flag in that case.
> >
> > I would rather not :P
> >
> > We can copy on write without a struct page behind the source today, no?
> > What is insufficient for the XIP code with the current COW?
>
> I've looked at the -mm version of mm/memory.c today, with intend to
> try out VM_PFNMAP for our xip mappings and replace nopage() with fault().
> The thing is, I believe it does'nt work for us:
>   * The way we recognize those mappings is through the rules set up
>   * by "remap_pfn_range()": the vma will have the VM_PFNMAP bit set,
>   * and the vm_pgoff will point to the first PFN mapped: thus every
>   * page that is a raw mapping will always honor the rule
>   *
>   *      pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >>
> PAGE_SHIFT)
>
> This is, as far as I can tell, not true for our xip mappings. Ext2 may
> spread the physical pages behind a given file all over its media. That
> means, that the pfns of the pages that form a vma may be more or less
> random rather than contiguous. The common memory management code
> cannot tell whether or not a given page has been COW'ed.
> Did I miss something?

I agree, the conditions imposed by the remap_pfn_range() don't work.
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