On Sat 12-01-19 21:17:46, Pankaj Gupta wrote: > > > > Right. Thinking about this I would be more concerned about the fact that > > > > guest can effectively pin amount of host's page cache upto size of the > > > > device/file passed to guest as PMEM, can't it Pankaj? Or is there some > > > > QEMU > > > > magic that avoids this? > > > > > > Yes, guest will pin these host page cache pages using 'get_user_pages' by > > > elevating the page reference count. But these pages can be reclaimed by > > > host > > > at any time when there is memory pressure. > > > > Wait, how can the guest pin the host pages? I would expect this to > > happen only when using vfio and device assignment. Otherwise, no the > > host can't reclaim a pinned page, that's the whole point of a pin to > > prevent the mm from reclaiming ownership. > > yes. You are right I just used the pin word but it does not actually pin pages > permanently. I had gone through the discussion on existing problems with > get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP > pin pages so I also used the word 'pin'. But guest does not permanently pin > these pages and these pages can be reclaimed by host. OK, then I was just confused how virtio-pmem is going to work. Thanks for explanation! So can I imagine this as guest mmaping the host file and providing the mapped range as "NVDIMM pages" to the kernel inside the guest? Or is it more complex? Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
