Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

[Ryan Roberts <ryan.roberts@arm.com>]

On 19/03/2024 09:20, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@arm.com> writes:
> 
>>>>> I agree phones are not the only platform. But Rome wasn't built in a
>>>>> day. I can only get
>>>>> started on a hardware which I can easily reach and have enough hardware/test
>>>>> resources on it. So we may take the first step which can be applied on
>>>>> a real product
>>>>> and improve its performance, and step by step, we broaden it and make it
>>>>> widely useful to various areas  in which I can't reach :-)
>>>>
>>>> We must guarantee the normal swap path runs correctly and has no
>>>> performance regression when developing SWP_SYNCHRONOUS_IO optimization.
>>>> So we have to put some effort on the normal path test anyway.
>>>>
>>>>> so probably we can have a sysfs "enable" entry with default "n" or
>>>>> have a maximum
>>>>> swap-in order as Ryan's suggestion [1] at the beginning,
>>>>>
>>>>> "
>>>>> So in the common case, swap-in will pull in the same size of folio as was
>>>>> swapped-out. Is that definitely the right policy for all folio sizes? Certainly
>>>>> it makes sense for "small" large folios (e.g. up to 64K IMHO). But I'm not sure
>>>>> it makes sense for 2M THP; As the size increases the chances of actually needing
>>>>> all of the folio reduces so chances are we are wasting IO. There are similar
>>>>> arguments for CoW, where we currently copy 1 page per fault - it probably makes
>>>>> sense to copy the whole folio up to a certain size.
>>>>> "
>>
>> I thought about this a bit more. No clear conclusions, but hoped this might help
>> the discussion around policy:
>>
>> The decision about the size of the THP is made at first fault, with some help
>> from user space and in future we might make decisions to split based on
>> munmap/mremap/etc hints. In an ideal world, the fact that we have had to swap
>> the THP out at some point in its lifetime should not impact on its size. It's
>> just being moved around in the system and the reason for our original decision
>> should still hold.
>>
>> So from that PoV, it would be good to swap-in to the same size that was
>> swapped-out.
> 
> Sorry, I don't agree with this.  It's better to swap-in and swap-out in
> smallest size if the page is only accessed seldom to avoid to waste
> memory.

If we want to optimize only for memory consumption, I'm sure there are many
things we would do differently. We need to find a balance between memory and
performance. The benefits of folios are well documented and the kernel is
heading in the direction of managing memory in variable-sized blocks. So I don't
think it's as simple as saying we should always swap-in the smallest possible
amount of memory.

You also said we should swap *out* in smallest size possible. Have I
misunderstood you? I thought the case for swapping-out a whole folio without
splitting was well established and non-controversial?

> 
>> But we only kind-of keep that information around, via the swap
>> entry contiguity and alignment. With that scheme it is possible that multiple
>> virtually adjacent but not physically contiguous folios get swapped-out to
>> adjacent swap slot ranges and then they would be swapped-in to a single, larger
>> folio. This is not ideal, and I think it would be valuable to try to maintain
>> the original folio size information with the swap slot. One way to do this would
>> be to store the original order for which the cluster was allocated in the
>> cluster. Then we at least know that a given swap slot is either for a folio of
>> that order or an order-0 folio (due to cluster exhaustion/scanning). Can we
>> steal a bit from swap_map to determine which case it is? Or are there better
>> approaches?
> 
> [snip]
> 
> --
> Best Regards,
> Huang, Ying