On Mon, 2018-12-03 at 13:05 -0800, Dan Williams wrote: > On Mon, Dec 3, 2018 at 12:53 PM Alexander Duyck > <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote: > > > > 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. > > Until p2pdma there are no struct pages for device memory, is that what > you're referring? Honestly I am not sure. It is possible I am getting beyond my depth. My understanding is that we have a 'struct page' for any of these pages that we are currently using. It is just a matter of if we want to pass the struct page around or not. So for example in the case of an MMIO page we still have a 'struct page', however the PG_reserved flag is set on such a page, so KVM is opting to not touch the reference count, modify the dirty/accessed bits, and is generally reducing performance as a result. > Otherwise any device driver that leaks "struct pages" into random code > paths in the kernel had better not expect to be able to > surprise-remove those pages from the system. Any dev_pagemap user > should expect to do a coordinated removal with the driver that waits > for page references to drop before the device can be physically > removed. Right. This part I get. However I would imagine there still has to be some exception handling in the case of a PCIe backed region of memory so that if the device falls of the bus we clean up the dev_pagemap memory. > > Holding a reference to the page doesn't guarantee that the backing > > device cannot go away. > > Correct there is no physical guarantee, but that's not the point. It > needs to be coordinated, otherwise all bets are off with respect to > system stability. Right. > > 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. > > Is it? MMIO does not typically have a corresponding 'struct page'. I think we might be talking about different things when we say 'struct page'. I'm pretty sure there has to be a 'struct page' for the MMIO region as otherwise we wouldn't be able to check for the PG_reserved bit in the 'struct page'. Do you maybe mean that MMIO doesn't have a corresponding virtual address or TLB entry? I know that is what we are normally generating via the ioremap family of calls in device drivers in order to access such memory if I am not mistaken. > > I believe this is also why > > MEMORY_DEVICE_PUBLIC specifically calls out that you should not allow > > pinning such memory. > > I don't think that call out was referencing device hotplug, I believe > it was the HMM expectation that it should be able to move an HMM page > from device to System-RAM at will. I could be wrong. If so that would make this patch set easier since essentially it would just mean that any PageReserved page that matches is_zone_device_page would fall into this category then and I could just drop patch 2, and probably combine the entire fix for all of this into one patch as it would only really be a few additional lines.
