Re: [PATCH Part2 RFC v4 10/40] x86/fault: Add support to handle the RMP fault for user address
From: Vlastimil Babka <hidden>
Date: 2021-07-30 16:00:23
Also in:
linux-coco, linux-crypto, linux-efi, linux-mm, lkml, platform-driver-x86
On 7/7/21 8:35 PM, Brijesh Singh wrote:
quoted hunk ↗ jump to hunk
--- a/mm/memory.c +++ b/mm/memory.c@@ -4407,6 +4407,15 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf) return 0; } +static int handle_split_page_fault(struct vm_fault *vmf) +{ + if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT)) + return VM_FAULT_SIGBUS; + + __split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL); + return 0; +} +
I think back in v1 Dave asked if khugepaged will just coalesce this back, and it wasn't ever answered AFAICS. I've checked the code and I think the answer is: no. Khugepaged isn't designed to coalesce a pte-mapped hugepage back to pmd in place. And the usual way (copy to a new huge page) I think will not succeed because IIRC the page is also FOLL_PIN pinned and khugepaged_scan_pmd() will see the elevated refcounts via is_refcount_suitable() and give up. So the lack of coalescing (in case the sub-page leading to split becomes guest private again later) is somewhat suboptimal, but not critical.