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x86/xen: update xen_oldmem_pfn_is_ram() documentation
After removing /dev/kmem, sanitizing /proc/kcore and handling /dev/mem, this series tackles the last sane way how a VM could accidentially access logically unplugged memory managed by a virtio-mem device: /proc/vmcore When dumping memory via "makedumpfile", PG_offline pages, used by virtio-mem to flag logically unplugged memory, are already properly excluded; however, especially when accessing/copying /proc/vmcore "the usual way", we can still end up reading logically unplugged memory part of a virtio-mem device. Patch #1-#3 are cleanups. Patch #4 extends the existing oldmem_pfn_is_ram mechanism. Patch #5-torvalds#7 are virtio-mem refactorings for patch torvalds#8, which implements the virtio-mem logic to query the state of device blocks. Patch torvalds#8: "Although virtio-mem currently supports reading unplugged memory in the hypervisor, this will change in the future, indicated to the device via a new feature flag. We similarly sanitized /proc/kcore access recently. [...] Distributions that support virtio-mem+kdump have to make sure that the virtio_mem module will be part of the kdump kernel or the kdump initrd; dracut was recently [2] extended to include virtio-mem in the generated initrd. As long as no special kdump kernels are used, this will automatically make sure that virtio-mem will be around in the kdump initrd and sanitize /proc/vmcore access -- with dracut" This is the last remaining bit to support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE [3] in the Linux implementation of virtio-mem. Note: this is best-effort. We'll never be able to control what runs inside the second kernel, really, but we also don't have to care: we only care about sane setups where we don't want our VM getting zapped once we touch the wrong memory location while dumping. While we usually expect sane setups to use "makedumfile", nothing really speaks against just copying /proc/vmcore, especially in environments where HWpoisioning isn't typically expected. Also, we really don't want to put all our trust completely on the memmap, so sanitizing also makes sense when just using "makedumpfile". [1] https://lkml.kernel.org/r/[email protected] [2] dracutdevs/dracut#1157 [3] https://lists.oasis-open.org/archives/virtio-comment/202109/msg00021.html This patch (of 9): The callback is only used for the vmcore nowadays. Link: https://lkml.kernel.org/r/[email protected] Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: David Hildenbrand <[email protected]> Reviewed-by: Boris Ostrovsky <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: "H. Peter Anvin" <[email protected]> Cc: Juergen Gross <[email protected]> Cc: Stefano Stabellini <[email protected]> Cc: "Michael S. Tsirkin" <[email protected]> Cc: Jason Wang <[email protected]> Cc: Dave Young <[email protected]> Cc: Baoquan He <[email protected]> Cc: Vivek Goyal <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Oscar Salvador <[email protected]> Cc: Mike Rapoport <[email protected]> Cc: "Rafael J. Wysocki" <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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