On Mon, May 01, 2017 at 01:19:24PM -0700, Dan Williams wrote: > On Mon, May 1, 2017 at 6:55 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote: > > On Mon, May 01, 2017 at 01:23:59PM +0300, Kirill A. Shutemov wrote: > >> On Sun, Apr 30, 2017 at 07:14:24PM -0400, Jerome Glisse wrote: > >> > On Sat, Apr 29, 2017 at 01:17:26PM +0300, Kirill A. Shutemov wrote: > >> > > On Fri, Apr 28, 2017 at 03:33:07PM -0400, Jerome Glisse wrote: > >> > > > On Fri, Apr 28, 2017 at 12:22:24PM -0700, Dan Williams wrote: > >> > > > > Are you sure about needing to hook the 2 -> 1 transition? Could we > >> > > > > change ZONE_DEVICE pages to not have an elevated reference count when > >> > > > > they are created so you can keep the HMM references out of the mm hot > >> > > > > path? > >> > > > > >> > > > 100% sure on that :) I need to callback into driver for 2->1 transition > >> > > > no way around that. If we change ZONE_DEVICE to not have an elevated > >> > > > reference count that you need to make a lot more change to mm so that > >> > > > ZONE_DEVICE is never use as fallback for memory allocation. Also need > >> > > > to make change to be sure that ZONE_DEVICE page never endup in one of > >> > > > the path that try to put them back on lru. There is a lot of place that > >> > > > would need to be updated and it would be highly intrusive and add a > >> > > > lot of special cases to other hot code path. > >> > > > >> > > Could you explain more on where the requirement comes from or point me to > >> > > where I can read about this. > >> > > > >> > > >> > HMM ZONE_DEVICE pages are use like other pages (anonymous or file back page) > >> > in _any_ vma. So i need to know when a page is freed ie either as result of > >> > unmap, exit or migration or anything that would free the memory. For zone > >> > device a page is free once its refcount reach 1 so i need to catch refcount > >> > transition from 2->1 > >> > >> What if we would rework zone device to have pages with refcount 0 at > >> start? > > > > That is a _lot_ of work from top of my head because it would need changes > > to a lot of places and likely more hot code path that simply adding some- > > thing to put_page() note that i only need something in put_page() i do not > > need anything in the get page path. Is adding a conditional branch for > > HMM pages in put_page() that much of a problem ? > > > > > >> > This is the only way i can inform the device that the page is now free. See > >> > > >> > https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-v21&id=52da8fe1a088b87b5321319add79e43b8372ed7d > >> > > >> > There is _no_ way around that. > >> > >> I'm still not convinced that it's impossible. > >> > >> Could you describe lifecycle for pages in case of HMM? > > > > Process malloc something, end it over to some function in the program > > that use the GPU that function call GPU API (OpenCL, CUDA, ...) that > > trigger a migration to device memory. > > > > So in the kernel you get a migration like any existing migration, > > original page is unmap, if refcount is all ok (no pin) then a device > > page is allocated and thing are migrated to device memory. > > > > What happen after is unknown. Either userspace/kernel driver decide > > to migrate back to system memory, either there is an munmap, either > > there is a CPU page fault, ... So from that point on the device page > > as the exact same life as a regular page. > > > > Above i describe the migrate case, but you can also have new memory > > allocation that directly allocate device memory. For instance if the > > GPU do a page fault on an address that isn't back by anything then > > we can directly allocate a device page. No migration involve in that > > case. > > > > HMM pages are like any other pages in most respect. Exception are: > > - no GUP > > - no KSM > > - no lru reclaim > > - no NUMA balancing > > - no regular migration (existing migrate_page) > > > > The fact that minimum refcount for ZONE_DEVICE is 1 already gives > > us for free most of the above exception. To convert the refcount to > > be like other pages would mean that all of the above would need to > > be audited and probably modify to ignore ZONE_DEVICE pages (i am > > pretty sure Dan do not want any of the above either). > > Right, adding HMM references to get_page() and put_page() seems less > intrusive. Given how uncommon HMM hardware is (insert grumble about no > visible upstream user of this functionality) I think the 'static > branch' approach helps mitigate the impact for everything else. > Looking back, I should have used that mechanism for the pmem use case, > but it's moot now. I do not need anything in get_page() all i need is something in put_page() to catch the 2 -> 1 refcount transition to know when a page is freed. Cheers, Jérôme -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
