On 2/11/2022 10:39 AM, David Hildenbrand wrote: > 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 >>> Acked-by: Felix Kuehling >>> Reviewed-by: Alistair Popple >> 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? Device coherent pages can be FOLL_PIN. However, we want to avoid FOLL_LONGTERM with these pages because that would affect our memory manager's ability to evict device memory. Regards, Alex Sierra >> >> >> ... 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.) Yes, that's correct. Device coherent pages are pte present. Regards, Alex Sierra >