On Thu, Jun 20, 2019 at 10:45:19PM +0300, Jarkko Sakkinen wrote: > On Wed, Jun 19, 2019 at 11:22:16AM -0700, Sean Christopherson wrote: > > Using per-vma refcounting to track mm_structs associated with an enclave > > requires hooking .vm_close(), which in turn prevents the mm from merging > > vmas (precisely to allow refcounting). > > > > Avoid refcounting encl_mm altogether by registering an mmu_notifier at > > .mmap(), removing the dying encl_mm at mmu_notifier.release() and > > protecting mm_list during reclaim via a per-enclave SRCU. > > > > Removing refcounting/vm_close() allows merging of enclave vmas, at the > > cost of delaying removal of encl_mm structs from mm_list, i.e. an mm is > > disassociated from an enclave when the mm exits or the enclave dies, as > > opposed to when the last vma (in a given mm) is closed. > > > > The impact of delying encl_mm removal is its memory footprint and > > whatever overhead is incurred during EPC reclaim (to walk an mm's vmas). > > Practically speaking, a stale encl_mm will exist for a meaningful amount > > of time if and only if the enclave is mapped in a long-lived process and > > then passed off to another long-lived process. It is expected that the > > vast majority of use cases will not encounter this condition, e.g. even > > using a daemon to build enclaves should not result in a stale encl_mm as > > the builder should never need to mmap() the enclave. > > > > Even if there are scenarios that lead to defunct encl_mms, the cost is > > likely far outweighed by the benefits of reducing the number of vmas > > across all enclaves. > > > > Note, using SRCU to protect mm_list is not strictly necessary, i.e. the > > existing walker with encl_mm refcounting could be massaged to work with > > mmu_notifier.release(), but the resulting code is subtle and fragile (I > > never actually got it working). The primary issue is that an encl_mm > > can't be moved off the list until its refcount goes to zero, otherwise > > the custom walker goes off into the weeds. The refcount requirement > > then prevents using mm_list to identify if an mmu_notifier.release() > > has fired, i.e. another mechanism is needed to guard against races > > between exit_mmap() and sgx_release(). > > > > Cc: Dave Hansen <dave.hansen@xxxxxxxxx> > > Cc: Andy Lutomirski <luto@xxxxxxxxxx> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > > OK, so what this needs is to split two patches. That can be seen from > your last paragraph (you are implying it with your note). Then this > can be sanely reviewed. > > And no, this does not fight by any means with the changes that I'm > doing. OK, so please ignore what I just said. You're right what I asked could not be sanely done :-) /Jarkko
