Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH 1/5] mm: Add support for unaccepted memory | Date | Tue, 17 Aug 2021 17:00:55 +0200 |
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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
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