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

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Message-ID: <54433b22-edaa-421e-9c99-6ee99734ab6d@redhat.com>
Date: Tue, 4 Mar 2025 11:21:37 +0100
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 cgroups@...r.kernel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-api@...r.kernel.org, "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, Jonathan Corbet <corbet@....net>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Muchun Song <muchun.song@...ux.dev>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v3 00/20] mm: MM owner tracking for large folios
 (!hugetlb) + CONFIG_NO_PAGE_MAPCOUNT

On 03.03.25 23:43, Andrew Morton wrote:
> On Mon,  3 Mar 2025 17:29:53 +0100 David Hildenbrand <david@...hat.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