Re: [PATCH v2 1/2] kernel/fork.c: get totalram_pages from memblock to calculate max_threads

From: David Hildenbrand
Date: Tue Jul 02 2024 - 03:02:46 EST


On 02.07.24 06:26, Andrew Morton wrote:
On Mon, 1 Jul 2024 01:34:09 +0000 Wei Yang <richard.weiyang@xxxxxxxxx> wrote:

Since we plan to move the accounting into __free_pages_core(),
totalram_pages may not represent the total usable pages on system
at this point when defer_init is enabled.

Yes, things like totalram_pages() are very old, and were a good idea at the
time, but things moved on.

Instead we can get the total usable pages from memblock directly.

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -44,6 +44,7 @@
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/mm_inline.h>
+#include <linux/memblock.h>
#include <linux/nsproxy.h>
#include <linux/capability.h>
#include <linux/cpu.h>
@@ -999,7 +1000,7 @@ void __init __weak arch_task_cache_init(void) { }
static void set_max_threads(unsigned int max_threads_suggested)
{
u64 threads;
- unsigned long nr_pages = totalram_pages();
+ unsigned long nr_pages = PHYS_PFN(memblock_phys_mem_size() - memblock_reserved_size());

The result of this subtraction has meaning. Even if it is only used
once, I suspect it should be in a standalone function which has
documentation which describes that meaning. Having fork.c make an
open-coded poke into memblock internals seems wrong, no?

I was just about the comment the same thing:

I'm starting to wonder if we should have a helper like

memblock_estimated_nr_pages()

or sth like that that abstracts this?

--
Cheers,

David / dhildenb