On Fri, May 19, 2023, Mike Rapoport wrote: > On Fri, May 19, 2023 at 08:40:48AM -0700, Sean Christopherson wrote: > > On Thu, Mar 09, 2023, Mike Rapoport wrote: > > > On Thu, Mar 09, 2023 at 01:59:00AM +0000, Edgecombe, Rick P wrote: > > > > On Wed, 2023-03-08 at 11:41 +0200, Mike Rapoport wrote: > > > > > From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx> > > > > > > > > > > Hi, > > > > > > > > > > This is a third attempt to make page allocator aware of the direct > > > > > map > > > > > layout and allow grouping of the pages that must be unmapped from > > > > > the direct map. > > > > > > > > > > This a new implementation of __GFP_UNMAPPED, kinda a follow up for > > > > > this set: > > > > > > > > > > https://lore.kernel.org/all/20220127085608.306306-1-rppt@xxxxxxxxxx > > > > > > > > > > but instead of using a migrate type to cache the unmapped pages, the > > > > > current implementation adds a dedicated cache to serve __GFP_UNMAPPED > > > > > allocations. > > > > > > > > It seems a downside to having a page allocator outside of _the_ page > > > > allocator is you don't get all of the features that are baked in there. > > > > For example does secretmem care about numa? I guess in this > > > > implementation there is just one big cache for all nodes. > > > > > > > > Probably most users would want __GFP_ZERO. Would secretmem care about > > > > __GFP_ACCOUNT? > > > > > > The intention was that the pages in cache are always zeroed, so __GFP_ZERO > > > is always implicitly there, at least should have been. > > > > Would it be possible to drop that assumption/requirement, i.e. allow allocation of > > __GFP_UNMAPPED without __GFP_ZERO? At a glance, __GFP_UNMAPPED looks like it would > > be a great fit for backing guest memory, in particular for confidential VMs. And > > for some flavors of CoCo, i.e. TDX, the trusted intermediary is responsible for > > zeroing/initializing guest memory as the untrusted host (kernel/KVM) doesn't have > > access to the guest's encryption key. In other words, zeroing in the kernel would > > be unnecessary work. > > Making and unmapped allocation without __GFP_ZERO shouldn't be a problem. > > However, using a gfp flag and hooking up into the free path in page > allocator have issues and preferably should be avoided. > > Will something like unmapped_alloc() and unmapped_free() work for your > usecase? Yep, I'm leaning more and more towards having KVM implement its own ioctl() for managing this type of memory. Wiring that up to use dedicated APIs should be no problem. Thanks!
