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.) -- Thanks, David / dhildenb
