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

[Ryan Roberts]

On 04/03/2024 21:57, Barry Song wrote:
> On Tue, Mar 5, 2024 at 1:21 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> Hi Barry,
>>
>> On 04/03/2024 10:37, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@xxxxxxxx>
>>>
>>> page_vma_mapped_walk() within try_to_unmap_one() races with other
>>> PTEs modification such as break-before-make, while iterating PTEs
>>> of a large folio, it will only begin to acquire PTL after it gets
>>> a valid(present) PTE. break-before-make intermediately sets PTEs
>>> to pte_none. Thus, a large folio's PTEs might be partially skipped
>>> in try_to_unmap_one().
>>
>> I just want to check my understanding here - I think the problem occurs for
>> PTE-mapped, PMD-sized folios as well as smaller-than-PMD-size large folios? Now
>> that I've had a look at the code and have a better understanding, I think that
>> must be the case? And therefore this problem exists independently of my work to
>> support swap-out of mTHP? (From your previous report I was under the impression
>> that it only affected mTHP).
> 
> I think this affects all large folios with PTEs entries more than 1. but hugeTLB
> is handled as a whole in try_to_unmap_one and its rmap is removed all
> together, i feel hugeTLB doesn't have this problem.
> 
>>
>> Its just that the problem is becoming more pronounced because with mTHP,
>> PTE-mapped large folios are much more common?
> 
> right. as now large folios become a more common case, and it is my case
> running in millions of phones.
> 
> BTW, I feel we can somehow learn from hugeTLB, for example, we can reclaim
> all PTEs all together rather than iterating PTEs one by one. This will improve
> performance. for example, a batched
> set_ptes_to_swap_entries()
> {
> }
> then we only need to loop once for a large folio, right now we are looping
> nr_pages times.

You still need a pte-pte loop somewhere. In hugetlb's case it's in the arch
implementation. HugeTLB ptes are all a fixed size for a given VMA, which makes
things a bit easier too, whereas in the regular mm, they are now a variable size.

David and I introduced folio_pte_batch() to help gather batches of ptes, and it
uses the contpte bit to avoid iterating over intermediate ptes. And I'm adding
swap_pte_batch() which does a similar thing for swap entry batching in v4 of my
swap-out series.

For your set_ptes_to_swap_entries() example, I'm not sure what it would do other
than loop over the PTEs setting an incremented swap entry to each one? How is
that more performant?

Thanks,
Ryan