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Date: Mon, 3 Mar 2025 14:43:32 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
 cgroups@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
 linux-api@vger.kernel.org, "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>, Johannes
 Weiner <hannes@cmpxchg.org>, Michal =?ISO-8859-1?Q?Koutn=FD?=
 <mkoutny@suse.com>, Jonathan Corbet <corbet@lwn.net>, Andy Lutomirski
 <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar
 <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave Hansen
 <dave.hansen@linux.intel.com>, Muchun Song <muchun.song@linux.dev>,
 "Liam R. Howlett" <Liam.Howlett@oracle.com>, Lorenzo Stoakes
 <lorenzo.stoakes@oracle.com>, Vlastimil Babka <vbabka@suse.cz>, Jann Horn
 <jannh@google.com>
Subject: Re: [PATCH v3 00/20] mm: MM owner tracking for large folios
 (!hugetlb) + CONFIG_NO_PAGE_MAPCOUNT
Message-Id: <20250303144332.4cb51677966b515ee0c89a44@linux-foundation.org>
In-Reply-To: <20250303163014.1128035-1-david@redhat.com>
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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.

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.