On 01.08.24 10:16, Dev Jain wrote:
I and Ryan had a discussion and we thought it would be best to get feedback from the community. The migration mm selftest currently fails on arm64 for shared anon mappings, due to the following race:
Do you mean MAP_SHARED|MAP_ANON or MAP_PRIVATE|MAP_ANON_fork? Because you note shmem below, I assume you mean MAP_SHARED|MAP_ANON
Migration: Page fault: try_to_migrate_one(): handle_pte_fault(): 1. Nuke the PTE PTE has been deleted => do_pte_missing() 2. Mark the PTE for migration PTE has not been deleted but is just not "present" => do_swap_page()
In filemap_fault_recheck_pte_none() we recheck under PTL to make sure that a temporary pte_none() really was persistent pte_none() and not a temporary pte_none() under PTL.
Should we do something similar in do_fault()? I see that we already do something like that on the "!vma->vm_ops->fault" path.
But of course, there is a tradeoff between letting migration (temporarily) fail and grabbing the PTL during page faults.
-- Cheers, David / dhildenb
