Race condition observed between page migration and page fault handling on arm64 machines

[David Hildenbrand]

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