Race condition observed between page migration and page fault handling on arm64 machines
Dev Jain
dev.jain at arm.com
Thu Aug 1 02:38:30 PDT 2024
On 8/1/24 14:12, David Hildenbrand wrote:
> 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
Yes.
>
>>
>> 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().
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