Re: [PATCH v3 00/20] mm: MM owner tracking for large folios (!hugetlb) + CONFIG_NO_PAGE_MAPCOUNT

David Hildenbrand
2025-03-04T10:21:48Z

On 03.03.25 23:43, Andrew Morton wrote:
> On Mon, 3 Mar 2025 17:29:53 +0100 David Hildenbrand <david@redhat.com> wrote:
>
>> Some smaller change based on Zi Yan's feedback (thanks!).
>>
>>
>> Let's add an "easy" way to decide -- without false positives, without
>> page-mapcounts and without page table/rmap scanning -- whether a large
>> folio is "certainly mapped exclusively" into a single MM, or whether it
>> "maybe mapped shared" into multiple MMs.
>>
>> Use that information to implement Copy-on-Write reuse, to convert
>> folio_likely_mapped_shared() to folio_maybe_mapped_share(), and to
>> introduce a kernel config option that let's us not use+maintain
>> per-page mapcounts in large folios anymore.
>>
>> ...
>>
>> The goal is to make CONFIG_NO_PAGE_MAPCOUNT the default at some point,
>> to then slowly make it the only option, as we learn about real-life
>> impacts and possible ways to mitigate them.
>
> I expect that we'll get very little runtime testing this way, and we
> won't hear about that testing unless there's a failure.
>
> Part of me wants to make it default on right now, but that's perhaps a
> bit mean to linux-next testers.

Yes, letting this sit at least for some time before we enable it in
linux-next as default might make sense.

> > Or perhaps default-off for now and switch to default-y for 6.15-rcX?

Maybe default-off for now, until we rebase mm-unstable to 6.15-rcX.
Then, default on first in linux-next, and then upstream (6.16).

>
> I suggest this just to push things along more aggressively - we may
> choose to return to default-off after a few weeks of -rcX.

Yeah, I'm usually very careful; sometimes a bit too careful :)

--
Cheers,

David / dhildenb