On Mon, Oct 3, 2011 at 9:04 AM, Nai Xia <nai.xia@xxxxxxxxx> wrote: > Hi, > > On Sunday 02 October 2011 17:43:23 Prateek Sharma wrote: >> Hello , >> I came across the dirty-page tracking patch here: >> [https://lkml.org/lkml/2011/6/21/169] . >> There is some mention of dirty-bit tracking not working with EPT. Can >> someone please clarify this? Does this patch only work when using >> software shadow page tables ? > > When ksm is scanning the memory, it is working on the host virutal > addresses, so this patch tries to figure out if the page pointed to by > the pte is actually dirty in the guests through the scanning of sptes. > But, Intel EPT does not update the dirty bit (and other similar bits) > in sptes, so for Intel EPT we fall back to checksum based approach > in the patch. > > This patch should work on AMD RVI/NPT although I hadn't the hardware > to test it yet. And also, my v2 patch considers huge page as a whole, and > try to skip it if any of the subpage is dirty, and I am still trying to > quantify the cost balance between breaking up active huge pages and > potential swapping. So maybe sometime I will give out a v3 patch totally > disabling the huge page consideration. > >> >> On a related note (i've asked a related question on this list a >> couple of days back), why can we not use the >> kvm_vm_ioctl_get_dirty_log function > > While I am not an expert in kvm, I don't think kvm_vm_ioctl_get_dirty_log() > or kvm_memory_slot->dirty_bitmap can catch the normal writes issued from inside > guest OS. Maybe some experts in this list can help you understand the code ;-) > Thanks for responding! I always wondered why KSM doesnt use the dirty page info and had started making some changes until i came across your patch :) . Without the dirty-page logging KSM just cannot scale - scanning 100s of GB memory can take hours! I am a bit confused about how live migration can proceed if qemu doesnt have accurate guest dirty info, file-backed page or not. AFAIK Xen toggles the read-only bits in its shadow tables during the migration to get a dirty-bitmap. Does KVM do the same? My question mainly is: how does KVM acquire the dirty-page info for migration when using EPT and the like (ie no shadow MMU). Thanks! > Thanks, > > Nai > > >> [http://lxr.linux.no/#linux+v3.0/arch/x86/kvm/x86.c#L3297] ? Is it >> because it is too expensive? If yes, can we use the dirty-page >> tracking ideas in this patch and use it for the migration code? >> >> >> Thanks for reading, >> Prateek >> > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
