On Wed, Dec 11, 2024 at 2:51 PM Russell King (Oracle) <linux@xxxxxxxxxxxxxxx> wrote: > On Wed, Dec 11, 2024 at 02:32:51PM +0100, Linus Walleij wrote: > > On Tue, Dec 10, 2024 at 5:06 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > > > > From: Arnd Bergmann <arnd@xxxxxxxx> > > > > > > CONFIG_HIGHPTE was added in linux-2.6.32, a few years before 64-bit > > > support. At the time it made sense, as the CONFIG_ARM_LPAE option allowed > > > systems with 16GB of memory that made lowmem a particularly scarce > > > resource, and the HIGHPTE implementation gave feature parity with 32-bit > > > x86 and frv machines. > > > > > > Since Arm is the last architecture remaining that uses this, and almost > > > no 32-bit machines support more than 4GB of RAM, the cost of continuing > > > to maintain HIGHPTE seems unjustified, so remove it here to allow > > > simplifying the generic page table handling. > > > > > > Link: https://lore.kernel.org/lkml/20241204103042.1904639-8-arnd@xxxxxxxxxx/T/#u > > > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> > > > > I'm in favor of this if the x86 patch goes in. We need to get rid > > of highmem anyway and this will need to happen sooner or later > > either way. > > Well... I use highmem routinely. Oh I don't mean we should get rid of it without any replacement. Certainly systems with big physical memories need to be usable. I am pursuing two ideas (inspired by Arnd and MM people): 1. The easy option - "densemem", on systems with a "hole" in the physical memory making the 1:1 linear phys-to-virt map run out too soon and overconsume virual memory, actually collect the physical memory on low virtual addresses by elaborate phys-to-virt virt-to-phys and page numbering that isn't 1:1. 2. The hard option - 4G-by-4G splitting, making the kernel and userspace virtual memory spaces separate as it is in hardware on S/390, so the kernel can use a while 4G of memory for its needs. I banged my head against this a fair amount of time, so I might be incompetent to do it, but I still try. Yours, Linus Walleij
