On Thu, Apr 04, 2024, David Hildenbrand wrote: > On 04.04.24 00:19, Sean Christopherson wrote: > > Hmm, we essentially already have an mmu_notifier today, since secondary MMUs need > > to be invalidated before consuming dirty status. Isn't the end result essentially > > a sane FOLL_TOUCH? > > Likely. As stated in my first mail, FOLL_TOUCH is a bit of a mess right now. > > Having something that makes sure the writable PTE/PMD is dirty (or > alternatively sets it dirty), paired with MMU notifiers notifying on any > mkclean would be one option that would leave handling how to handle dirtying > of folios completely to the core. It would behave just like a CPU writing to > the page table, which would set the pte dirty. > > Of course, if frequent clearing of the dirty PTE/PMD bit would be a problem > (like we discussed for the accessed bit), that would not be an option. But > from what I recall, only clearing the PTE/PMD dirty bit is rather rare. And AFAICT, all cases already invalidate secondary MMUs anyways, so if anything it would probably be a net positive, e.g. the notification could more precisely say that SPTEs need to be read-only, not blasted away completely.
