On Sun, Apr 30, 2017 at 4:14 PM, Jerome Glisse <jglisse@xxxxxxxxxx> 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 > > 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. Ok, but I need to point out that this not a ZONE_DEVICE requirement. This is an HMM-specific need. So, this extra reference counting should be clearly delineated as part of the MEMORY_DEVICE_PRIVATE use case. Can we hide the extra reference counting behind a static branch so that the common case fast path doesn't get slower until a HMM device shows up? -- 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>
