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

[Andrew Morton]

On Mon,  3 Mar 2025 17:29:53 +0100 David Hildenbrand <david@xxxxxxxxxx> 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.

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

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