On 8/1/24 14:12, David Hildenbrand wrote:

On 01.08.24 10:16, Dev Jain wrote:

quoted

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

Yes.

quoted

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.

To dampen the tradeoff, we could do this in shmem_fault() instead? But 
then, this would mean that we do this in all

kinds of vma->vm_ops->fault, only when we discover another reference 
count race condition :) Doing this in do_fault()

should solve this once and for all. In fact, do_pte_missing() may call 
do_anonymous_page() or do_fault(), and I just

noticed that the former already checks this using vmf_pte_changed().