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kalloc: never-used memory stays a range; 256 GB; free is a counter

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Every page of RAM went onto the free list at boot, which means writing
each one -- a link, and kfree()'s 4 KB of junk -- before the first
process ran: 64 GB of memset on a 64 GB machine to say it was free.
Memory nobody has used yet is kept as a cursor and an end per region
now, and kalloc() takes from those once the list of given-back pages
is empty. A page is first touched by whoever asked for it.

That makes the cap on RAM cheap to raise, and PHYSTOP_MAX goes from 64
to 256 GB, which still leaves 248 GB of the kernel PDPT for device
windows. MEMREGION_MAX goes from 32 to 64 for the firmware maps that
cut RAM into many pieces.

kalloc_free_bytes() walked the whole free list under the allocator's
lock -- two million links on 8 GB, every second top or fastfetch was
running, with every CPU that wanted a page waiting behind it. It reads
a counter now. And the buffer cache, which keeps what it grows to, is
reported as Buffers in /proc/meminfo and sysinfo(2) rather than as
memory used by nothing.

Checked under QEMU at 1, 2 and 6 GB: MemFree right, zsh splitting
200000 words, fork loops, a 74 MB write.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
This commit is contained in:
Nexoniarz 2026-10-23 14:05:40 +02:00
commit b70e36e0a2
4 changed files with 106 additions and 23 deletions
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12
include/memlayout.h
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@ -27,7 +27,7 @@ extern uint64 phystop;
// PHYSTOP still means what it always did -- the top of what kalloc may hand
// out, and the bound kfree() rejects pages above -- so every existing call
// site is unaffected.
#define MEMREGION_MAX 32
#define MEMREGION_MAX 64
#ifndef __ASSEMBLER__
struct memregion {
@ -57,10 +57,14 @@ extern uint64 lowtop;
//
// kvmalloc() now maps as many 1 GB pages as the machine has RAM, so the limit
// is the page table rather than a constant: the kernel PDPT has 512 entries
// and ioremap takes what the direct map does not, so 64 GB of RAM still leaves
// 448 GB of device window. A machine with more than this gets the rest
// and ioremap takes what the direct map does not, so 256 GB of RAM still
// leaves 248 GB of device window. A machine with more than this gets the rest
// ignored, with a line saying so.
#define PHYSTOP_MAX (64ULL << 30)
//
// It was 64 GB while handing memory to the allocator meant writing every page
// of it at boot. kalloc.c keeps never-used memory as ranges now, so a bigger
// machine costs nothing until its memory is used.
#define PHYSTOP_MAX (256ULL << 30)
// What the *loader's* page tables cover, and therefore the ceiling on anything
// that runs before kvmalloc() builds the kernel's own.