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

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> 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.

> 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