On 11.02.22 18:07, Felix Kuehling wrote: > > Am 2022-02-11 um 11:39 schrieb David Hildenbrand: >> On 11.02.22 17:15, David Hildenbrand wrote: >>> On 01.02.22 16:48, Alex Sierra wrote: >>>> Device memory that is cache coherent from device and CPU point of view. >>>> This is used on platforms that have an advanced system bus (like CAPI >>>> or CXL). Any page of a process can be migrated to such memory. However, >>>> no one should be allowed to pin such memory so that it can always be >>>> evicted. >>>> >>>> Signed-off-by: Alex Sierra <alex.sierra@xxxxxxx> >>>> Acked-by: Felix Kuehling <Felix.Kuehling@xxxxxxx> >>>> Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx> >>> So, I'm currently messing with PageAnon() pages and CoW semantics ... >>> all these PageAnon() ZONE_DEVICE variants don't necessarily make my life >>> easier but I'm not sure yet if they make my life harder. I hope you can >>> help me understand some of that stuff. >>> >>> 1) What are expected CoW semantics for DEVICE_COHERENT? >>> >>> I assume we'll share them just like other PageAnon() pages during fork() >>> readable, and the first sharer writing to them receives an "ordinary" >>> !ZONE_DEVICE copy. >>> >>> So this would be just like DEVICE_EXCLUSIVE CoW handling I assume, just >>> that we don't have to go through the loop of restoring a device >>> exclusive entry? >>> >>> 2) How are these pages freed to clear/invalidate PageAnon() ? >>> >>> I assume for PageAnon() ZONE_DEVICE pages we'll always for via >>> free_devmap_managed_page(), correct? >>> >>> >>> 3) FOLL_PIN >>> >>> While you write "no one should be allowed to pin such memory", patch #2 >>> only blocks FOLL_LONGTERM. So I assume we allow ordinary FOLL_PIN and >>> you might want to be a bit more precise? >>> >>> >>> ... I'm pretty sure we cannot FOLL_PIN DEVICE_PRIVATE pages, but can we >>> FILL_PIN DEVICE_EXCLUSIVE pages? I strongly assume so? >>> >>> >>> Thanks for any information. >>> >> (digging a bit more, I realized that device exclusive pages are not >> actually/necessarily ZONE_DEVICE pages -- so I assume DEVICE_COHERENT >> will be the actual first PageAnon() ZONE_DEVICE pages that can be >> present in a page table.) > > I think DEVICE_GENERIC pages can also be mapped in the page table. In > fact, the first version of our patches attempted to add migration > support to DEVICE_GENERIC. But we were convinced to create a new > ZONE_DEVICE page type for our use case instead. Do you know if DEVICE_GENERIC pages would end up as PageAnon()? My assumption was that they would be part of a special mapping. -- Thanks, David / dhildenb
