On Mon, May 02, 2016 at 05:22:49PM +0200, Jerome Glisse wrote: > I think this is still fine as it means that device will read only and thus > you can migrate to different page (ie the guest is not expecting to read back > anything writen by the device and device writting to the page would be illegal > and a proper IOMMU would forbid it). So it is like direct-io when you write > from anonymous memory to a file. Agreed. write=1 is so that if there's an O_DIRECT write() and the app is only reading, there will be no COW generated on shared anonymous memory/MAP_PRIVATE-filebacked. > Now that i think again about it, i don't think it exist. pmdp_collapse_flush() > will flush the tlb and thus send an IPI but get_user_pages_fast() can't be > preempted so the flush will have to wait for existing get_user_pages_fast() to > complete. Or am i missunderstanding flush ? So khugepaged is safe from GUP_fast > point of view like the comment, inside it, says. This is exactly correct, there's no race window. The IPI (or the quiescent point in case of the gup_fast RCU version) are the things that flush away get_user_pages_fast with pmdp_collapse_flush(). > Well you can't not rely on special vma here. Qemu alloc anonymous memory and > hand it over to guest, then a guest driver (ie runing in the guest not on the > host) try to map that memory and need valid DMA address for it, this is when > vfio (on the host kernel) starts pining memory of regular anonymous vma (on > the host). That same memory might back some special vma with ->mmap callback > but in the guest. Point is there is no driver on the host and no special vma. > From host point of view this is anonymous memory, but from guest POV it is > just memory. It's quite important it stays regular tmpfs/anon as device memory is managed by the device and we'd lose everything (KSM/swapping/NUMA balancing/compaction/memory-hotunplug/CMA etc..). -- 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>