Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before swap_free()

David Hildenbrand
2023-05-17T08:33:42Z

>>> Is there any other way of handling this? E.g. not release the metadata
>>> in arch_swap_invalidate_page() but later in set_pte_at() once it was
>>> restored. But then we may leak this metadata if there's no set_pte_at()
>>> (the process mapping the swap entry died).
>>
>> That was my immediate thought: do we really have to hook into
>> swap_range_free() at all?
>
> As I alluded to in another reply, without the hook in
> swap_range_free() I think we would either end up with a race or an
> effective memory leak in the arch code that maintains the metadata for
> swapped out pages, as there would be no way for the arch-specific code
> to know when it is safe to free it after swapin.

Agreed, hooking swap_range_free() is actually cleaner (also considering
COW-shared pages).

>
>> And I also wondered why we have to do this
>> from set_pte_at() and not do this explicitly (maybe that's the other
>> arch_* callback on the swapin path).
>
> I don't think it's necessary, as the set_pte_at() call sites for
> swapped in pages are known. I'd much rather do this via an explicit
> hook at those call sites, as the existing approach of implicit
> restoring seems too subtle and easy to be overlooked when refactoring,
> as we have seen with this bug. In the end we only have 3 call sites
> for the hook and hopefully the comments that I'm adding are sufficient
> to ensure that any new swapin code should end up with a call to the
> hook in the right place.

Agreed, much cleaner, thanks!

--
Thanks,

David / dhildenb