Messages in this thread | | | Date | Tue, 11 Apr 2006 22:03:27 +0200 | From | Carsten Otte <> | Subject | Re: [patch] do_no_pfn handler |
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Linus Torvalds wrote: > You _really_ cannot do COW together with "random pfn filling". I still have'nt found a good way to do so, even after discussing with Nick and Hugh, but that's exactly where I intend to get for the xip stuff.
Today, the _only_ code that uses the struct page behind those DCSS segments is aops->nopage (as return value) and do_wp_page. Those small servers have almost no local memory (kernel, libraries, and binaries are shared), and the mem_map array is a large overhead.
> You can do COW with a pure remap_pfn_range() (ie a /dev/mem kind of > mapping, or a frame buffer etc), but that's only because it has a very > magic special case that is used to distinguish between cow'ed pages and > the pages that were inserted initially. > > We have no free bits in the page tables to say "this is a COW page" in > general (on x86 we could do it, but some other architectures don't have > any SW-usable bits). That's true. One can store that information in the vma flags, and split the vma into 3 vmas once we have a write fault. Although that would work in theory, I doubt it would save lot of memory because of too many vmas, and I think we would burn precious CPU horsepower walking all those vmas. I believe Hugh has already done an implementation for that which he does not consider nice. I have not found a feasible way to adress that issue so far, and I promise to keep from coding until I find a reasonable non-intrusive way to get there. --
Carsten Otte IBM Linux technology center ARCH=s390 - 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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