memblock: fix memblock_estimated_nr_free_pages() for soft-reserved memory

memblock_estimated_nr_free_pages() returns the difference between the total
size of the "memory" memblock type and the "reserved" memblock type.

The "soft-reserved" memory regions are added to the "reserved" memblock
type, but not to the "memory" memblock type. Therefore,
memblock_estimated_nr_free_pages() may return a smaller value than
expected, or if it underflows, an extremely large value.

/proc/sys/kernel/threads-max is determined by the value of
memblock_estimated_nr_free_pages().  This issue was discovered on machines
with CXL memory because kernel.threads-max was either smaller than expected
or extremely large for the installed DRAM size.

This fixes the issue by replacing memblock_reserved_size() with
memblock_reserved_kern_size() that tells how much memory was
reserved from the actual RAM.

Suggested-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://patch.msgid.link/20251111010010.7800-1-akinobu.mita@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: mita

mm/memblock.c

@@ -1826,7 +1826,8 @@ phys_addr_t __init_memblock memblock_reserved_kern_size(phys_addr_t limit, int n
  */
 unsigned long __init memblock_estimated_nr_free_pages(void)
 {
-	return PHYS_PFN(memblock_phys_mem_size() - memblock_reserved_size());
+	return PHYS_PFN(memblock_phys_mem_size() -
+			memblock_reserved_kern_size(MEMBLOCK_ALLOC_ANYWHERE, NUMA_NO_NODE));
 }
 
 /* lowest address */