Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio

David Hildenbrand
2024-03-04T21:15:12Z

>>> Do we need a Fixes tag?
>
> I am not quite sure which commit should be here for a fixes tag.
> I think it's more of an optimization.

Good, that helps!

>
>>>
>>
>> What would be the description of the problem we are fixing?
>>
>> 1) failing to unmap?
>>
>> That can happen with small folios as well IIUC.
>>
>> 2) Putting the large folio on the deferred split queue?
>>
>> That sounds more reasonable.
>
> I don't feel it is reasonable. Avoiding this kind of accident splitting
> from the kernel's improper code is a more reasonable approach
> as there is always a price to pay for splitting and unfolding PTEs
> etc.
>
> While we can't avoid splitting coming from userspace's
> MADV_DONTNEED, munmap, mprotect, we have a way
> to ensure the kernel itself doesn't accidently break up a
> large folio.

Note that on the next vmscan we would retry, find the remaining present
entries and swapout that thing completely :)

>
> In OPPO's phones, we ran into some weird bugs due to skipped PTEs
> in try_to_unmap_one. hardly could we fix it from the root cause. with
> various races, figuring out their timings was really a big pain :-)
>

I can imagine. I assume, though, that it might be related to the way the
cont-pte bit was handled. Ryan's implementation should be able to cope
with that.

> But we did "resolve" those bugs by entirely untouching all PTEs if we
> found some PTEs were skipped in try_to_unmap_one [1].
>
> While we find we only get the PTL from 2nd, 3rd but not
> 1st PTE, we entirely give up on try_to_unmap_one, and leave
> all PTEs untouched.
>
> /* we are not starting from head */
> if (!IS_ALIGNED((unsigned long)pvmw.pte, CONT_PTES * sizeof(*pvmw.pte))) {
> ret = false;
> atomic64_inc(&perf_stat.mapped_walk_start_from_non_head);
> set_pte_at(mm, address, pvmw.pte, pteval);
> page_vma_mapped_walk_done(&pvmw);
> break;
> }
> This will ensure all PTEs still have a unified state such as CONT-PTE
> after try_to_unmap fails.
> I feel this could have some false postive because when racing
> with unmap, 1st PTE might really become pte_none. So explicitly
> holding PTL from 1st PTE seems a better way.

Can we estimate the "cost" of holding the PTL?

--
Cheers,

David / dhildenb