On 13.08.21 16:49, Joerg Roedel wrote:
Hi Dave, On Thu, Aug 12, 2021 at 07:14:20AM -0700, Dave Hansen wrote:maybe_accept_page() { unsigned long huge_pfn = page_to_phys(page) / PMD_SIZE; /* Test the bit before taking any locks: */ if (test_bit(huge_pfn, &accepted_bitmap)) return; spin_lock_irq(); /* Retest inside the lock: */ if (test_bit(huge_pfn, &accepted_bitmap)) return; tdx_accept_page(page, PMD_SIZE); set_bit(huge_pfn, &accepted_bitmap)); spin_unlock_irq(); }Yeah, this could work, but the global lock is likely the show-stopper here. For SNP we also not allowed to double-validate, so we need something that basically indicates 'validation-is-ongoing' on a per 2MB basis. I am not an mm expert, but a page flag probably doesn't work. The flag would be on the head of the 2MB range and when that page is already used somewhere else there is no guarantee that the flag will survive. But correct me if I am wrong here :) The other options I can come up with are not great either: 1) using an AVL bit in the direct-mapping PMD of that page. The page-table would only be walked if the bit in the accept_bitmap is clear. But I am not sure that all memory which needs to be validated is in the direct-map. 2) Use another page-sized bitmap. If the machine has more than 64GB of memory the bit index is wrapped around. This shouldn't be a performance problem at runtime, if this page is only consulted when the valid bit is clear in the accept_bitmap. MM experts could certainly come up with better ideas :)
Not sure if already discussed, but what about making sure that free pages are not a mixture (partially unaccepted, partially accepted).
You'd have to expose the pages in that granularity to the buddy (__free_pages_core), indicating the state. You'd have to reject merging pages of differing acceptance state.
Accepting a page would then be handled outside of the zone lock, completely controlled by the state.
So a page in the buddy would either be completely accepted or completely unaccepted, signaled e.g., by PageOffline().
Consequently, when allocating a 4KiB page, you'd split an unaccepted 2MiB page into separate unaccepted pages. You'd grab one of the unaccepted 4KiB pages and accept it before initializing it and handing it out.
-- Thanks, David / dhildenb
