Re: [PATCH v4 4/6] mm: Introduce a pageflag for partially mapped folios

[Usama Arif]

On 20/08/2024 17:30, Barry Song wrote:

> Hi Usama,
> thanks! I can't judge if we need this partially_mapped flag. but if we
> need, the code
> looks correct to me. I'd like to leave this to David and other experts to ack.
> 

Thanks for the reviews!

> an alternative approach might be two lists? one for entirely_mapped,
> the other one
> for split_deferred. also seems ugly ?
> 

That was my very first prototype! I shifted to using a bool which I sent in v1, and then a bit in _flags_1 as David suggested. I believe a bit in _flags_1 is the best way forward, as it leaves the most space in folio for future work.

> On the other hand, when we want to extend your patchset to mTHP other than PMD-
> order, will the only deferred_list create huge lock contention while
> adding or removing
> folios from it?
> 

Yes, I would imagine so. the deferred_split_queue is per memcg/node, so that helps.

Also, this work is tied to khugepaged. So would need some thought when doing it for mTHP.

I would imagine doing underused shrinker for mTHP would be less beneficial compared to doing it for 2M THP. But probably needs experimentation.

Thanks