Messages in this thread | | | From | Bjorn Helgaas <> | Subject | Re: [patch] do_no_pfn | Date | Tue, 20 Jun 2006 10:03:52 -0600 |
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On Tuesday 20 June 2006 02:01, Jes Sorensen wrote: > Robin Holt wrote: > > On Mon, Jun 19, 2006 at 03:06:05PM +0200, Andi Kleen wrote: > >> The big question is - why do you have pages without struct page? > >> It seems ... wrong. > ... > Note that Bjorn Helgas has a case where he needs this as well.
I do have a case where I used pages without struct pages, but I don't really like the implementation, and I'd love to have someone who knows about VM tell me "no, dummy, you should do it this way instead."
Here's the scenario: I'm trying to implement /sys/class/pci_bus/DDDD:BB/legacy_mem so we can run X servers on multiple VGA cards. The chipset (used in HP parisc and ia64 boxes) supports multiple PCI root bridges, and it routes the VGA legacy MMIO space at 0xA0000-0xBFFFF to one of them.
This region is MMIO, so there are no struct pages for it. I can easily mmap the space for the first VGA device. But to support a second device, I have to be able to invalidate the mappings for the first device, twiddle stuff in the chipset, and make new mappings for the second device. And of course I have to do the reverse (invalidate mappings of second device, twiddle chipset, map first device) when the first X server faults on the frame buffer.
Basically, only one of the /sys/class/pci_bus/DDDD:BB/legacy_mem files can have an active mmap at a time, and I haven't figured out a good way to do the mutual exclusion.
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