Messages in this thread | | | Date | Fri, 15 Jun 2007 14:46:07 -0700 | From | "Jared Hulbert" <> | Subject | Re: [PATCH 2.6.21] cramfs: add cramfs Linear XIP |
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On 6/15/07, Carsten Otte <cotte@de.ibm.com> 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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