On Mon, 2018-12-03 at 12:31 -0800, Dan Williams wrote: > On Mon, Dec 3, 2018 at 12:21 PM Alexander Duyck > <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote: > > > > On Mon, 2018-12-03 at 11:47 -0800, Dan Williams wrote: > > > On Mon, Dec 3, 2018 at 11:25 AM Alexander Duyck > > > <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote: > > > > > > > > Add a means of exposing if a pagemap supports refcount pinning. I am doing > > > > this to expose if a given pagemap has backing struct pages that will allow > > > > for the reference count of the page to be incremented to lock the page > > > > into place. > > > > > > > > The KVM code already has several spots where it was trying to use a > > > > pfn_valid check combined with a PageReserved check to determien if it could > > > > take a reference on the page. I am adding this check so in the case of the > > > > page having the reserved flag checked we can check the pagemap for the page > > > > to determine if we might fall into the special DAX case. > > > > > > > > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> > > > > --- > > > > drivers/nvdimm/pfn_devs.c | 2 ++ > > > > include/linux/memremap.h | 5 ++++- > > > > include/linux/mm.h | 11 +++++++++++ > > > > 3 files changed, 17 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c > > > > index 6f22272e8d80..7a4a85bcf7f4 100644 > > > > --- a/drivers/nvdimm/pfn_devs.c > > > > +++ b/drivers/nvdimm/pfn_devs.c > > > > @@ -640,6 +640,8 @@ static int __nvdimm_setup_pfn(struct nd_pfn *nd_pfn, struct dev_pagemap *pgmap) > > > > } else > > > > return -ENXIO; > > > > > > > > + pgmap->support_refcount_pinning = true; > > > > + > > > > > > There should be no dev_pagemap instance instance where this isn't > > > true, so I'm missing why this is needed? > > > > I thought in the case of HMM there were instances where you couldn't > > pin the page, isn't there? Specifically I am thinking of the definition > > of MEMORY_DEVICE_PUBLIC: > > Device memory that is cache coherent from device and CPU point of > > view. This is use on platform that have an advance system bus (like > > CAPI or CCIX). A driver can hotplug the device memory using > > ZONE_DEVICE and with that memory type. Any page of a process can be > > migrated to such memory. However no one should be allow to pin such > > memory so that it can always be evicted. > > > > It sounds like MEMORY_DEVICE_PUBLIC and MMIO would want to fall into > > the same category here in order to allow a hot-plug event to remove the > > device and take the memory with it, or is my understanding on this not > > correct? > > I don't understand how HMM expects to enforce no pinning, but in any > event it should always be the expectation an elevated reference count > on a page prevents that page from disappearing. Anything else is > broken. I don't think that is true for device MMIO though. In the case of MMIO you have the memory region backed by a device, if that device is hot-plugged or fails in some way then that backing would go away and the reads would return and all 1's response. Holding a reference to the page doesn't guarantee that the backing device cannot go away. I believe that is the origin of the original use of the PageReserved check in KVM in terms of if it will try to use the get_page/put_page functions. I believe this is also why MEMORY_DEVICE_PUBLIC specifically calls out that you should not allow pinning such memory. - Alex
