On 2018年08月10日 21:27, David Hildenbrand wrote: > On 09.08.2018 12:52, Zhang Yi wrote: >> For device specific memory space, when we move these area of pfn to >> memory zone, we will set the page reserved flag at that time, some of >> these reserved for device mmio, and some of these are not, such as >> NVDIMM pmem. >> >> Now, we map these dev_dax or fs_dax pages to kvm for DIMM/NVDIMM >> backend, since these pages are reserved. the check of >> kvm_is_reserved_pfn() misconceives those pages as MMIO. Therefor, we >> introduce 2 page map types, MEMORY_DEVICE_FS_DAX/MEMORY_DEVICE_DEV_DAX, >> to indentify these pages are from NVDIMM pmem. and let kvm treat these >> as normal pages. >> >> Without this patch, Many operations will be missed due to this >> mistreatment to pmem pages. For example, a page may not have chance to >> be unpinned for KVM guest(in kvm_release_pfn_clean); not able to be >> marked as dirty/accessed(in kvm_set_pfn_dirty/accessed) etc. >> > I am right now looking into (and trying to better document) PG_reserved > - and having a hard time :) . > > One of the main points about reserved pages is that the struct pages are > not to be touched. See [1] (I know that statement is fairly old, but it > resembles what PG_reserved is actually used for nowadays - with some > exceptions unfortunately.). > > Struct pages part of user space tables that are PG_reserved can indicate > (as of now according to my research) > - MMIO pages > - Selected MMAPed pages - e.g. vDSO > - Zero page > - PMEM pages as you correctly state > > So I wonder, if it is really the right approach to silently go ahead and > treat reserved pages just like they would not be reserved. Maybe the > right approach would rather be to do something about pmem pages being > reserved. Yes, they are never to be given to the page allocator, but I > wonder if PG_reserved is strictly needed for that. > > [1] https://lists.linuxcoding.com/kernel/2005-q3/msg10350.html Thanks David list the long history of Page reserved, By now, I think we treat nvdimm as a device not a DRAM, also has it's device driver which manager its own device memory. From this perspective, it is reasonable to set these pages as zone device memory and mark reserved flag. @Dan @Dave, how do you think about this? > >> V1: >> https://lkml.org/lkml/2018/7/4/91 >> >> V2: >> https://lkml.org/lkml/2018/7/10/135 >> >> V3: >> [PATCH V3 1/4] Needs Comments. >> [PATCH V3 2/4] Update the description of MEMORY_DEVICE_DEV_DAX: Jan >> [PATCH V3 3/4] Acked-by: Jan in V2 >> [PATCH V3 4/4] Needs Comments. >> >> Zhang Yi (4): >> kvm: remove redundant reserved page check >> mm: introduce memory type MEMORY_DEVICE_DEV_DAX >> mm: add a function to differentiate the pages is from DAX device >> memory >> kvm: add a check if pfn is from NVDIMM pmem. >> >> drivers/dax/pmem.c | 1 + >> include/linux/memremap.h | 8 ++++++++ >> include/linux/mm.h | 12 ++++++++++++ >> virt/kvm/kvm_main.c | 16 ++++++++-------- >> 4 files changed, 29 insertions(+), 8 deletions(-) >> >
