From: Dave Hansen <dave.hansen@intel•com>
To: Arnd Bergmann <arnd@arndb•de>, Arnd Bergmann <arnd@kernel•org>,
linux-mm@kvack•org
Cc: Andrew Morton <akpm@linux-foundation•org>,
Andreas Larsson <andreas@gaisler•com>,
Christophe Leroy <chleroy@kernel•org>,
Dave Hansen <dave.hansen@linux•intel.com>,
Jason Gunthorpe <jgg@nvidia•com>,
Linus Walleij <linus.walleij@linaro•org>,
Matthew Wilcox <willy@infradead•org>,
Richard Weinberger <richard@nod•at>,
Russell King <linux@armlinux•org.uk>,
linux-arm-kernel@lists•infradead.org,
linux-fsdevel@vger•kernel.org, linuxppc-dev@lists•ozlabs.org,
x86@kernel•org, Thomas Gleixner <tglx@linutronix•de>,
Ingo Molnar <mingo@redhat•com>, Borislav Petkov <bp@alien8•de>,
"H. Peter Anvin" <hpa@zytor•com>,
Madhavan Srinivasan <maddy@linux•ibm.com>,
Michael Ellerman <mpe@ellerman•id.au>,
Nicholas Piggin <npiggin@gmail•com>,
Michal Simek <monstr@monstr•eu>,
"David Hildenbrand (Red Hat)" <david@kernel•org>,
Lorenzo Stoakes <lorenzo.stoakes@oracle•com>,
"Liam R. Howlett" <Liam.Howlett@oracle•com>,
Vlastimil Babka <vbabka@suse•cz>, Mike Rapoport <rppt@kernel•org>,
Suren Baghdasaryan <surenb@google•com>,
Michal Hocko <mhocko@suse•com>, Nishanth Menon <nm@ti•com>,
Lucas Stach <l.stach@pengutronix•de>
Subject: Re: [PATCH 1/4] arch/*: increase lowmem size to avoid highmem use
Date: Fri, 19 Dec 2025 12:52:21 -0800 [thread overview]
Message-ID: <a2ce2849-e572-404c-9713-9283a43c09fe@intel.com> (raw)
In-Reply-To: <bad18ad8-93e8-4150-a85e-a2852e243363@app.fastmail.com>
On 12/19/25 12:20, Arnd Bergmann wrote:
>> For simplicity, I think this can just be:
>>
>> - default VMSPLIT_3G
>> + default VMSPLIT_2G
>>
>> I doubt the 2G vs. 2G_OPT matters in very many cases. If it does, folks
>> can just set it in their config manually.
>>
>> But, in the end, I don't this this matters all that much. If you think
>> having x86 be consistent with ARM, for example, is more important and
>> ARM really wants this complexity, I can live with it.
> Yes, I think we do want the default of VMSPLIT_3G_OPT for
> configs that have neither highmem nor lpae, otherwise the most
> common embedded configs go from 3072 MiB to 1792 MiB of virtual
> addressing, and that is much more likely to cause regressions
> than the 2816 MiB default.
>
> It would be nice to not need the VMSPLIT_2G default for PAE/LPAE,
> but that seems like a larger change.
The only thing we'd "regress" would be someone who is repeatedly
starting from scratch with a defconfig and expecting defconfig to be the
same all the time. I honestly think that's highly unlikely.
If folks are upgrading and _actually_ exposed to regressions, they've
got an existing config and won't be hit by these defaults at *all*. They
won't actually regress.
In other words, I think we can be a lot more aggressive about defaults
than with the feature set we support. I'd much rather add complexity in
here for solving a real problem, like if we have armies of 32-bit x86
users constantly starting new projects from scratch and using defconfigs.
I'd _really_ like to keep the defaults as simple as possible.
next prev parent reply other threads:[~2025-12-19 20:52 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-19 16:15 [PATCH 0/4] mm: increase lowmem size in linux-7.0 Arnd Bergmann
2025-12-19 16:15 ` [PATCH 1/4] arch/*: increase lowmem size to avoid highmem use Arnd Bergmann
2025-12-19 18:02 ` Dave Hansen
2025-12-19 20:20 ` Arnd Bergmann
2025-12-19 20:52 ` Dave Hansen [this message]
2025-12-20 12:17 ` Arnd Bergmann
2026-01-06 17:01 ` Dave Hansen
2025-12-21 9:30 ` David Hildenbrand (Red Hat)
2025-12-21 15:26 ` H. Peter Anvin
2025-12-24 11:35 ` Christophe Leroy (CS GROUP)
2026-01-08 14:32 ` Christophe Leroy (CS GROUP)
2025-12-19 16:15 ` [PATCH 2/4] ARM: add CONFIG_VMSPLIT_2G_OPT option Arnd Bergmann
2025-12-19 16:15 ` [PATCH 3/4] ARM: remove support for highmem on VIVT Arnd Bergmann
2025-12-19 17:14 ` Jason Gunthorpe
2025-12-19 20:34 ` Arnd Bergmann
2025-12-24 2:26 ` Jason Gunthorpe
2025-12-24 10:39 ` Arnd Bergmann
2026-01-02 12:12 ` Russell King (Oracle)
2025-12-19 16:15 ` [PATCH 4/4] mm: remove ARCH_NEEDS_KMAP_HIGH_GET Arnd Bergmann
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