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Thu, 18 Sep 2025 09:13:38 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: ksummit@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: AneuvEk2E7Kg Date: Thu, 18 Sep 2025 15:12:46 +0200 From: "Arnd Bergmann" To: "Jason Gunthorpe" Cc: ksummit@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, imx@lists.linux.dev, "Christophe Leroy" , "Richard Weinberger" , "Lucas Stach" , "Linus Walleij" , "Geert Uytterhoeven" , "Ankur Arora" , "David Hildenbrand" , "Mike Rapoport" , "Lorenzo Stoakes" , "Matthew Wilcox" , "Andrew Morton" , "Liam R. Howlett" , "Vlastimil Babka" , "Suren Baghdasaryan" , "Ira Weiny" , "Nishanth Menon" , =?UTF-8?Q?Heiko_St=C3=BCbner?= , "Alexander Sverdlin" , "Chester A. Unal" , "Sergio Paracuellos" , "Andreas Larsson" Message-Id: <02b0f383-1c43-4eeb-a76f-830c2970b833@app.fastmail.com> In-Reply-To: <20250917125951.GA1390993@nvidia.com> References: <4ff89b72-03ff-4447-9d21-dd6a5fe1550f@app.fastmail.com> <20250917125951.GA1390993@nvidia.com> Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout Content-Type: text/plain Content-Transfer-Encoding: 7bit On Wed, Sep 17, 2025, at 14:59, Jason Gunthorpe wrote: > On Tue, Sep 09, 2025 at 11:23:37PM +0200, Arnd Bergmann wrote: > >> I'm still collecting information about which of the remaining highmem >> users plan to keep updating their kernels and for what reason. > > On this topic of removing some parts of highmem, can we say goodbye to > kmap_high_get()? Only ARM uses it and only for > !cache_is_vipt_nonaliasing() systems. Good idea. I think we are almost there, just need to verify that there is actually no impact for existing users. I already knew that there is very little highmem usage on ARMv6 and earlier, but I tried to recheck all platforms that might be affected: * As you say (and contrary to what the code comments suggest), all ARMv7 and most ARMv6 systems have non-aliasing VIPT or PIPT caches, so kmap_high_get() does nothing, and we could just turn it off for those configurations. * ARMv6 with aliasing VIPT caches exist but are fairly rare, and I think there are no ARMv6 machines left that actually use highmem. AST2500 is commonly used and has aliasing caches, but I only see machines with 512MB or less, and its aspeed_v5_defconfig enables CONFIG_SMP (for AST2600). OMAP2 and realview are in the same category but less important. imx3, s3c64xx and bcm2835 are non-aliasing (and don't have highmem either). * VIVT caches are used on all ARMv5 and earlier, so removing kmap_high_get() would effectively break highmem these. Most ARMv5 use very small amounts of memory (<256M) in a single physical address range, so they are unaffected, we just have to go through the ones that are left. * Intel IOP (armv5 xscale) certainly had highmem but was removed a while ago. * Marvell mv78xx0 and kirkwood (armv5 pj1) were fairly powerful in 2008 and could support at least 1GB of RAM, but I only found one machine (OpenBlocks A7) that does this. It's unclear if anyone is still updating kernels on this machine, but they could /probably/ use VMSPLIT_3G_OPT if they do. * Microchip SAM9x7 is the newest ARMv5 chip, clearly does get kernel updates, and the only one I can think of with DDR3 support, but seems to be limited to 256MB total memory. * The Gemini (ARMv4) platform enabled highmem as part of commit c12d7e9fe9af ("ARM: defconfig: Update Gemini defconfig"), but I don't see why, as the machine cited there only has 128MB of RAM in a contiguous chunk. * A few ARMv4/v5 machines (omap1, davinci, ep93xx, clps71xx, sa1100, riscpc) used to select CONFIG_ARCH_HAS_HOLES_MEMORYMODEL or SPARSEMEM, which would indicate that they might need highmem even for <1GB configurations if their memory banks are far apart in physical memory. I checked those and as far as I can tell, they are always within 768MB or less. Arnd