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

[Barry Song <baohua@kernel.org>]

On Thu, Aug 22, 2024 at 3:04 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> 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.
>

Thanks for the explanation; I missed the earlier discussion.

> > 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.
>

Right.

> 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.
>

Agreed. I suspect this will be also useful for larger mTHPs like 1M and 512KB.
While lock contention might be a concern, it shouldn't be a barrier to
proceeding
with your patchset, which only supports PMD-order.

> Thanks
>

Thanks
Barry